January 2018 - tz - lists.iana.org

[PROPOSED] Add notes about Ghana confusion
by Paul Eggert Jan. 30, 2018

Jan. 30, 2018

* africa (Ghana): Add commentary about what little is known about time near Accra from 1919 through 1942. Every source seems to differ. For now, leave the data alone. It would be nice if someone could get to the bottom of this. --- africa | 31 +++++++++++++++++++++++++------ 1 file changed, 25 insertions(+), 6 deletions(-) diff --git a/africa b/africa index 02115ad..6e86d21 100644 --- a/africa +++ b/africa @@ -370,13 +370,32 @@ Zone Africa/Cairo 2:05:09 - LMT 1900 Oct # See Africa/Abidjan. # Ghana -# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S + +# From Paul Eggert (2019-01-30): # Whitman says DST was observed from 1931 to "the present"; -# Shanks & Pottenger say 1936 to 1942; -# and September 1 to January 1 is given by: -# Scott Keltie J, Epstein M (eds), The Statesman's Year-Book, -# 57th ed. Macmillan, London (1920), OCLC 609408015, pp xxviii. -# For lack of better info, assume DST was observed from 1920 to 1942. +# Shanks & Pottenger say 1936 to 1942 with 20 minutes of DST, +# with transitions on 09-01 and 12-31 at 00:00. +# Page 33 of Parish GCB, Colonial Reports - Annual. No. 1066. Gold +# Coast. Report for 1919. (March 1921), OCLC 784024077 +# http://libsysdigi.library.illinois.edu/ilharvest/africana/books2011-05/5530… +# lists the Determination of the Time Ordinance, 1919, No. 18, +# "to advance the time observed locally by the space of twenty minutes +# during the last four months of each year; the object in view being +# to extend during those months the period of daylight-time available +# for evening recreation after office hours." +# Vanessa Ogle, The Global Transformation of Time, 1870-1950 (2015), p 33, +# writes "In 1919, the Gold Coast (Ghana as of 1957) made Greenwich +# time its legal time and simultaneously legalized a summer time of +# UTC - 00:20 minutes from March to October."; a footnote lists +# the ordinance as being dated 1919-11-24. +# The Crown Colonist, Volume 12 (1942), p 176, says "the Government +# intend advancing Gold Coast time half an hour ahead of G.M.T. +# The actual date of the alteration has not yet been announced." +# These sources are incomplete and contradictory. Possibly what is +# now Ghana observed different DST regimes in different years. For +# lack of better info, use Shanks except treat the minus sign as a +# typo, and assume DST started in 1920 not 1936. +# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S Rule Ghana 1920 1942 - Sep 1 0:00 0:20 GHST Rule Ghana 1920 1942 - Dec 31 0:00 0 GMT # Zone NAME GMTOFF RULES FORMAT [UNTIL] -- 2.14.3

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OpenJDK/CLDR/ICU/Joda issues with Ireland change
by Stephen Colebourne Jan. 29, 2018

Jan. 29, 2018

Let me try to summarize why the negative SAVE (Ireland change) matters to Java-based projects. there are two basic streams of code - old and new. The older OpenJDK time-zone code and ICU Java code both derive from the same original 20 year old source. The newer OpenJDK time-zone code derives indirectly from Joda-Time, which is still based on many principles from the older OpenJDK code. The ThreeTen-Backport project is an early version of the newer OpenJDK time-zone code. Thus, the vast majority of Java time-zone code is linked and follows a similar approach. The basic data stored is: - a "raw" offset that changes over time, also known as "standard" - a "daylight" offset that changes over time (either relative to the "raw" or absolute) - time-zone changes available from 1970 or earlier - only the *current* names of time-zones are available Some problems: 1) The older code has limits on the difference between the "raw" and "daylight" offset in `GregorianCalendar`. Specifically, the "DST_OFFSET" can only be from 0 to 2 hours. Negative SAVE values are not expected or supported. Note that the limits of 0 to 2 hours are publicly visible via the API `GregorianCalendar.getLeastMinimum()` and friends. 2) The current names of the time-zones is accessed via an array of a fixed order, roughly [long-std, short-std, long-daylight, short-daylight]. Whether the std or daylight text form should be returned is determined by a parameter - a boolean, named "daylight". The basic way this is determined is whether the offset and the"raw" offset are the same or not. This is also expressed in `TimeZone.inDaylightTime()` and `ZoneRules.isDaylightSavings()`, making any switch in the boolean user-visible. The problem here is that the Ireland change flips when the boolean is true from summer to winter. Whereas the content of the array has been stable for 20 years. In the problematic case I looked at, this means that the wrong textual description is returned. ie. in winter, the code has always accessed array elements 0 and 1, whereas if the boolean flag is swapped it will access elements 2 and 3. Remember that only the *current* names of the time-zones are available - so if the Ireland change happens, the time-zone name will necessarily be wrong either before the change occurs or after it. It is a similar problem as Yoshito Umaoka from CLDR expressed a clash between data and code. Here the data on names would clash with the boolean "daylight" flag. Fixing the code doesn't help in this scenario - there is no viable approach to fixing it that can work, as Java has a long history of backwards compatibility over decades, not a year (as is being talked about). Specifically, some of these libraries (Joda-Time, ThreeTen-Backport) run with new tzdb data on older JDK versions. It is the incompatibility in the boolean, where false no longer means winter that cannot be accepted. So, to summarize, even if those in the CLDR/ICU/Java world thought this change was meaningful and positive, backwards compatibility rules out making it. But, in fact the majority of the feedback on the topic from software library maintainers has been negative. I'm pretty sure that negative SAVE is going to be rejected by CLDR, ICU, OpenJDK, Joda-Time and ThreeTen-Backport permanently, and probably Android and Apple too based on their responses so far. As I've outlined, there is no way to meet the backwards compatibility requirements with it. A one year stay of execution won't make a difference in the practicality of the change. FWIW, I understand why some are attached to trying to express data in a "pure" form, but that was never the original intent of the project IMO. The project needs to get back to serving its downstream consumers who just want to know the time without continuous instability in the data format. Stephen PS, Microsoft .NET APIs also have an IsDaylightSavingTime() method: https://msdn.microsoft.com/en-us/library/bb460642(v=vs.110).aspx#Anchor_3

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[PATCH] Clean up theory.html formatting
by Paul Eggert Jan. 28, 2018

Jan. 28, 2018

* theory.html: Redo formatting of the HTML to make it more consistent. Add URLs and some markup. The intent is to improve presentation and make the HTML easier to edit, without changing anything substantive. --- theory.html | 1742 ++++++++++++++++++++++++++++++++--------------------------- 1 file changed, 943 insertions(+), 799 deletions(-) diff --git a/theory.html b/theory.html index 6f03832..ff82fd7 100644 --- a/theory.html +++ b/theory.html @@ -1,26 +1,20 @@ -<!DOCTYPE html> <html lang="en"> <head> <title>Theory and pragmatics of the tz code and data</title> <meta charset="UTF-8"> </head> - - <body> - <h1>Theory and pragmatics of the tz code and data</h1> +<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1> <h3>Outline</h3> <nav> <ul> - <li><a href="#scope">Scope of the tz database</a></li> + <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code> + database</a></li> <li><a href="#naming">Names of time zone rules</a></li> <li><a href="#abbreviations">Time zone abbreviations</a></li> - <li><a href="#accuracy">Accuracy of the tz database</a></li> + <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code> + database</a></li> <li><a href="#functions">Time and date functions</a></li> <li><a href="#stability">Interface stability</a></li> <li><a href="#calendar">Calendrical issues</a></li> @@ -28,20 +22,27 @@ </ul> </nav> - - <section> - <h2 id="scope">Scope of the tz database</h2> +<section> + <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2> <p> -The tz database attempts to record the history and predicted future of -all computer-based clocks that track civil time. To represent this -data, the world is partitioned into regions whose clocks all agree -about timestamps that occur after the somewhat-arbitrary cutoff point -of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, -the database records all known clock transitions, and labels the region -with a notable location. Although 1970 is a somewhat-arbitrary -cutoff, there are significant challenges to moving the cutoff earlier -even by a decade or two, due to the wide variety of local practices -before computer timekeeping became prevalent. +The <a +href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code> +database</a> attempts to record the history and predicted future of +all computer-based clocks that track civil time. +It organizes <a href="tz-link.html">time zone and daylight saving time +data</a> by partitioning the world into <a +href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones">regions</a> +whose clocks all agree about timestamps that occur after the of the <a +href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> +(1970-01-01 00:00:00 <a +href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr +title="Coordinated Universal Time">UTC</abbr></a>). +For each such region, the database records all known clock +transitions, and labels the region with a notable location. +Although 1970 is a somewhat-arbitrary cutoff, there are significant +challenges to moving the cutoff earlier even by a decade or two, due +to the wide variety of local practices before computer timekeeping +became prevalent. </p> <p> @@ -59,193 +60,204 @@ necessarily follow database guidelines. </p> <p> -As described below, reference source code for using the tz database is -also available. The tz code is upwards compatible with POSIX, an -international standard for UNIX-like systems. As of this writing, the -current edition of POSIX is: - <a href="http://pubs.opengroup.org/onlinepubs/9699919799/"> - The Open Group Base Specifications Issue 7</a>, - IEEE Std 1003.1-2008, 2016 Edition. +As described below, reference source code for using the +<code><abbr>tz</abbr></code> database is also available. +The <code><abbr>tz</abbr></code> code is upwards compatible with <a +href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international +standard for <a +href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems. +As of this writing, the current edition of POSIX is: <a +href="http://pubs.opengroup.org/onlinepubs/9699919799/"> The Open +Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2008, 2016 +Edition. </p> - </section> - - +</section> - <section> - <h2 id="naming">Names of time zone rules</h2> +<section> + <h2 id="naming">Names of time zone rules</h2> <p> Each of the database's time zone rules has a unique name. Inexperienced users are not expected to select these names unaided. Distributors should provide documentation and/or a simple selection interface that explains the names; for one example, see the 'tzselect' -program in the tz code. The -<a href="http://cldr.unicode.org/">Unicode Common Locale Data -Repository</a> contains data that may be useful for other -selection interfaces. +program in the <code><abbr>tz</abbr></code> code. +The <a href="http://cldr.unicode.org/">Unicode Common Locale Data +Repository</a> contains data that may be useful for other selection +interfaces. </p> <p> The time zone rule naming conventions attempt to strike a balance among the following goals: </p> + <ul> <li> - Uniquely identify every region where clocks have agreed since 1970. - This is essential for the intended use: static clocks keeping local - civil time. + Uniquely identify every region where clocks have agreed since 1970. + This is essential for the intended use: static clocks keeping local + civil time. </li> <li> - Indicate to experts where that region is. + Indicate to experts where that region is. </li> <li> - Be robust in the presence of political changes. For example, names - of countries are ordinarily not used, to avoid incompatibilities - when countries change their name (e.g. Zaire→Congo) or when - locations change countries (e.g. Hong Kong from UK colony to - China). + Be robust in the presence of political changes. + For example, names of countries are ordinarily not used, to avoid + incompatibilities when countries change their name (e.g., + Zaire→Congo) or when locations change countries (e.g., Hong + Kong from UK colony to China). </li> <li> - Be portable to a wide variety of implementations. + Be portable to a wide variety of implementations. </li> <li> - Use a consistent naming conventions over the entire world. + Use a consistent naming conventions over the entire world. </li> </ul> + <p> -Names normally have the -form <var>AREA</var><code>/</code><var>LOCATION</var>, -where <var>AREA</var> is the name of a continent or ocean, -and <var>LOCATION</var> is the name of a specific -location within that region. North and South America share the same -area, '<code>America</code>'. Typical names are -'<code>Africa/Cairo</code>', '<code>America/New_York</code>', and -'<code>Pacific/Honolulu</code>'. +Names normally have the form +<var>AREA</var><code>/</code><var>LOCATION</var>, where +<var>AREA</var> is the name of a continent or ocean, and +<var>LOCATION</var> is the name of a specific location within that +region. +North and South America share the same area, '<code>America</code>'. +Typical names are '<code>Africa/Cairo</code>', +'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'. </p> <p> Here are the general rules used for choosing location names, in decreasing order of importance: </p> + <ul> <li> - Use only valid POSIX file name components (i.e., the parts of - names other than '<code>/</code>'). Do not use the file name - components '<code>.</code>' and '<code>..</code>'. - Within a file name component, - use only ASCII letters, '<code>.</code>', - '<code>-</code>' and '<code>_</code>'. Do not use - digits, as that might create an ambiguity with POSIX - TZ strings. A file name component must not exceed 14 - characters or start with '<code>-</code>'. E.g., - prefer '<code>Brunei</code>' to - '<code>Bandar_Seri_Begawan</code>'. Exceptions: see - the discussion - of legacy names below. + Use only valid POSIX file name components (i.e., the parts of + names other than '<code>/</code>'). + Do not use the file name components '<code>.</code>' and + '<code>..</code>'. + Within a file name component, use only <a + href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, + '<code>.</code>', '<code>-</code>' and '<code>_</code>'. + Do not use digits, as that might create an ambiguity with <a + href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag…">POSIX + <code>TZ</code> strings</a>. + A file name component must not exceed 14 characters or start with + '<code>-</code>'. + E.g., prefer '<code>Brunei</code>' to '<code>Bandar_Seri_Begawan</code>'. + Exceptions: see the discussion of legacy names below. </li> <li> - A name must not be empty, or contain '<code>//</code>', or - start or end with '<code>/</code>'. + A name must not be empty, or contain '<code>//</code>', or + start or end with '<code>/</code>'. </li> <li> - Do not use names that differ only in case. Although the reference - implementation is case-sensitive, some other implementations - are not, and they would mishandle names differing only in case. + Do not use names that differ only in case. + Although the reference implementation is case-sensitive, some + other implementations are not, and they would mishandle names + differing only in case. </li> <li> - If one name <var>A</var> is an initial prefix of another - name <var>AB</var> (ignoring case), then <var>B</var> - must not start with '<code>/</code>', as a - regular file cannot have - the same name as a directory in POSIX. For example, - '<code>America/New_York</code>' precludes - '<code>America/New_York/Bronx</code>'. + If one name <var>A</var> is an initial prefix of another + name <var>AB</var> (ignoring case), then <var>B</var> must not + start with '<code>/</code>', as a regular file cannot have the + same name as a directory in POSIX. + For example, '<code>America/New_York</code>' precludes + '<code>America/New_York/Bronx</code>'. </li> <li> - Uninhabited regions like the North Pole and Bouvet Island - do not need locations, since local time is not defined there. + Uninhabited regions like the North Pole and Bouvet Island + do not need locations, since local time is not defined there. </li> <li> - There should typically be at least one name for each ISO 3166-1 - officially assigned two-letter code for an inhabited country - or territory. + There should typically be at least one name for each <a + href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr + title="International Organization for Standardization">ISO</abbr> + 3166-1</a> officially assigned two-letter code for an inhabited + country or territory. </li> <li> - If all the clocks in a region have agreed since 1970, - don't bother to include more than one location - even if subregions' clocks disagreed before 1970. - Otherwise these tables would become annoyingly large. + If all the clocks in a region have agreed since 1970, + don't bother to include more than one location + even if subregions' clocks disagreed before 1970. + Otherwise these tables would become annoyingly large. </li> <li> - If a name is ambiguous, use a less ambiguous alternative; - e.g. many cities are named San José and Georgetown, so - prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'. + If a name is ambiguous, use a less ambiguous alternative; + e.g., many cities are named San José and Georgetown, so + prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and + '<code>Guyana</code>' to '<code>Georgetown</code>'. </li> <li> - Keep locations compact. Use cities or small islands, not countries - or regions, so that any future time zone changes do not split - locations into different time zones. E.g. prefer - '<code>Paris</code>' to '<code>France</code>', since - France has had multiple time zones. + Keep locations compact. + Use cities or small islands, not countries or regions, so that any + future time zone changes do not split locations into different + time zones. + E.g., prefer '<code>Paris</code>' to '<code>France</code>', since + <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France + has had multiple time zones</a>. </li> <li> - Use mainstream English spelling, e.g. prefer - '<code>Rome</code>' to '<code>Roma</code>', and prefer - '<code>Athens</code>' to the Greek - '<code>Αθήνα</code>' or the Romanized - '<code>Athína</code>'. - The POSIX file name restrictions encourage this rule. + Use mainstream English spelling, e.g., prefer '<code>Rome</code>' + to '<code>Roma</code>', and prefer '<code>Athens</code>' to the + Greek '<code>Αθήνα</code>' or the Romanized '<code>Athína</code>'. + The POSIX file name restrictions encourage this rule. </li> <li> - Use the most populous among locations in a zone, - e.g. prefer '<code>Shanghai</code>' to - '<code>Beijing</code>'. Among locations with - similar populations, pick the best-known location, - e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'. + Use the most populous among locations in a zone, + e.g., prefer '<code>Shanghai</code>' to + '<code>Beijing</code>'. + Among locations with similar populations, pick the best-known + location, e.g., prefer '<code>Rome</code>' to + '<code>Milan</code>'. </li> <li> - Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'. + Use the singular form, e.g., prefer '<code>Canary</code>' to + '<code>Canaries</code>'. </li> <li> - Omit common suffixes like '<code>_Islands</code>' and - '<code>_City</code>', unless that would lead to - ambiguity. E.g. prefer '<code>Cayman</code>' to - '<code>Cayman_Islands</code>' and - '<code>Guatemala</code>' to - '<code>Guatemala_City</code>', but prefer - '<code>Mexico_City</code>' to '<code>Mexico</code>' - because the country - of Mexico has several time zones. + Omit common suffixes like '<code>_Islands</code>' and + '<code>_City</code>', unless that would lead to ambiguity. + E.g., prefer '<code>Cayman</code>' to + '<code>Cayman_Islands</code>' and '<code>Guatemala</code>' to + '<code>Guatemala_City</code>', but prefer + '<code>Mexico_City</code>' to '<code>Mexico</code>' + because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the + country of Mexico has several time zones</a>. </li> <li> - Use '<code>_</code>' to represent a space. + Use '<code>_</code>' to represent a space. </li> <li> - Omit '<code>.</code>' from abbreviations in names, e.g. prefer - '<code>St_Helena</code>' to '<code>St._Helena</code>'. + Omit '<code>.</code>' from abbreviations in names. + E.g., prefer '<code>St_Helena</code>' to '<code>St._Helena</code>'. </li> <li> - Do not change established names if they only marginally - violate the above rules. For example, don't change - the existing name '<code>Rome</code>' to - '<code>Milan</code>' merely because - Milan's population has grown to be somewhat greater - than Rome's. + Do not change established names if they only marginally violate + the above rules. + For example, don't change the existing name '<code>Rome</code>' to + '<code>Milan</code>' merely because Milan's population has grown + to be somewhat greater than Rome's. </li> <li> - If a name is changed, put its old spelling in the - '<code>backward</code>' file. - This means old spellings will continue to work. + If a name is changed, put its old spelling in the + '<code>backward</code>' file. + This means old spellings will continue to work. </li> </ul> <p> The file '<code>zone1970.tab</code>' lists geographical locations used -to name time -zone rules. It is intended to be an exhaustive list of names for -geographic regions as described above; this is a subset of the names -in the data. Although a '<code>zone1970.tab</code>' location's longitude -corresponds to its LMT offset with one hour for every 15° east -longitude, this relationship is not exact. +to name time zone rules. +It is intended to be an exhaustive list of names for geographic +regions as described above; this is a subset of the names in the data. +Although a '<code>zone1970.tab</code>' location's +<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> +corresponds to +its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean +time (<abbr>LMT</abbr>)</a> offset with one hour for every 15° +east longitude, this relationship is not exact. </p> <p> @@ -254,864 +266,996 @@ and these older names are still supported. See the file '<code>backward</code>' for most of these older names (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). The other old-fashioned names still supported are -'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>'). +'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and +'<code>EET</code>' (see the file '<code>europe</code>'). </p> <p> Older versions of this package defined legacy names that are incompatible with the first rule of location names, but which are -still supported. These legacy names are mostly defined in the file -'<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names -'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the -legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'. +still supported. +These legacy names are mostly defined in the file +'<code>etcetera</code>'. +Also, the file '<code>backward</code>' defines the legacy names +'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', +and the file '<code>northamerica</code>' defines the legacy names +'<code>EST5EDT</code>', '<code>CST6CDT</code>', +'<code>MST7MDT</code>', and '<code>PST8PDT</code>'. </p> <p> -Excluding '<code>backward</code>' should not affect the other data. If -'<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the -remaining data. +Excluding '<code>backward</code>' should not affect the other data. +If '<code>backward</code>' is excluded, excluding +'<code>etcetera</code>' should not affect the remaining data. </p> +</section> - - </section> - <section> - <h2 id="abbreviations">Time zone abbreviations</h2> +<section> + <h2 id="abbreviations">Time zone abbreviations</h2> <p> When this package is installed, it generates time zone abbreviations like '<code>EST</code>' to be compatible with human tradition and POSIX. Here are the general rules used for choosing time zone abbreviations, in decreasing order of importance: +</p> + <ul> <li> - Use three to six characters that are ASCII alphanumerics or - '<code>+</code>' or '<code>-</code>'. - Previous editions of this database also used characters like - '<code> </code>' and '<code>?</code>', but these - characters have a special meaning to - the shell and cause commands like - '<code>set `date`</code>' - to have unexpected effects. - Previous editions of this rule required upper-case letters, - but the Congressman who introduced Chamorro Standard Time - preferred "ChST", so lower-case letters are now allowed. - Also, POSIX from 2001 on relaxed the rule to allow - '<code>-</code>', '<code>+</code>', - and alphanumeric characters from the portable character set - in the current locale. In practice ASCII alphanumerics and - '<code>+</code>' and '<code>-</code>' are safe in all locales. + Use three to six characters that are ASCII alphanumerics or + '<code>+</code>' or '<code>-</code>'. + Previous editions of this database also used characters like + '<code> </code>' and '<code>?</code>', but these characters have a + special meaning to the shell and cause commands like + '<code><a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a> + `<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>' + to have unexpected effects. + Previous editions of this rule required upper-case letters, but the + Congressman who + introduced <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro + Standard Time</a> preferred "ChST", so lower-case letters are now + allowed. + Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>', + '<code>+</code>', and alphanumeric characters from the portable + character set in the current locale. + In practice ASCII alphanumerics and '<code>+</code>' and + '<code>-</code>' are safe in all locales. - In other words, in the C locale the POSIX extended regular - expression <code>[-+[:alnum:]]{3,6}</code> should match - the abbreviation. - This guarantees that all abbreviations could have been - specified by a POSIX TZ string. - </li> - <li> - Use abbreviations that are in common use among English-speakers, - e.g. 'EST' for Eastern Standard Time in North America. - We assume that applications translate them to other languages - as part of the normal localization process; for example, - a French application might translate 'EST' to 'HNE'. - -<p><small>These abbreviations (for standard/daylight/etc. time) are: -ACST/ACDT Australian Central, -AST/ADT/APT/AWT/ADDT Atlantic, -AEST/AEDT Australian Eastern, -AHST/AHDT Alaska-Hawaii, -AKST/AKDT Alaska, -AWST/AWDT Australian Western, -BST/BDT Bering, -CAT/CAST Central Africa, -CET/CEST/CEMT Central European, -ChST Chamorro, -CST/CDT/CWT/CPT/CDDT Central [North America], -CST/CDT China, -GMT/BST/IST/BDST Greenwich, -EAT East Africa, -EST/EDT/EWT/EPT/EDDT Eastern [North America], -EET/EEST Eastern European, -GST Guam, -HST/HDT Hawaii, -HKT/HKST Hong Kong, -IST India, -IST/GMT Irish, -IST/IDT/IDDT Israel, -JST/JDT Japan, -KST/KDT Korea, -MET/MEST Middle European (a backward-compatibility alias for Central European), -MSK/MSD Moscow, -MST/MDT/MWT/MPT/MDDT Mountain, -NST/NDT/NWT/NPT/NDDT Newfoundland, -NST/NDT/NWT/NPT Nome, -NZMT/NZST New Zealand through 1945, -NZST/NZDT New Zealand 1946–present, -PKT/PKST Pakistan, -PST/PDT/PWT/PPT/PDDT Pacific, -SAST South Africa, -SST Samoa, -WAT/WAST West Africa, -WET/WEST/WEMT Western European, -WIB Waktu Indonesia Barat, -WIT Waktu Indonesia Timur, -WITA Waktu Indonesia Tengah, -YST/YDT/YWT/YPT/YDDT Yukon</small>.</p> - </li> - <li> - For zones whose times are taken from a city's longitude, use the -traditional <var>x</var>MT notation. The only abbreviation like this -in current use is 'GMT'. The others are for timestamps before 1960, -except that Monrovia Mean Time persisted until 1972. Typically, -numeric abbreviations (e.g., '<code>-</code>004430' for MMT) would -cause trouble here, as the numeric strings would exceed the POSIX length limit. + <p> + In other words, in the C locale the POSIX extended regular + expression <code>[-+[:alnum:]]{3,6}</code> should match the + abbreviation. + This guarantees that all abbreviations could have been specified by a + POSIX <code>TZ</code> string. + </p> + </li> + <li> + Use abbreviations that are in common use among English-speakers, + e.g., 'EST' for Eastern Standard Time in North America. + We assume that applications translate them to other languages + as part of the normal localization process; for example, + a French application might translate 'EST' to 'HNE'. -<p><small>These abbreviations are: -AMT Amsterdam, Asunción, Athens; -BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels, Bucharest; -CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; -DMT Dublin/Dunsink; -EMT Easter; -FFMT Fort-de-France; -FMT Funchal; -GMT Greenwich; -HMT Havana, Helsinki, Horta, Howrah; -IMT Irkutsk, Istanbul; -JMT Jerusalem; -KMT Kaunas, Kiev, Kingston; -LMT Lima, Lisbon, local, Luanda; -MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, Moratuwa, - Moscow; -PLMT Phù Liễn; -PMT Paramaribo, Paris, Perm, Pontianak, Prague; -PMMT Port Moresby; -QMT Quito; -RMT Rangoon, Riga, Rome; -SDMT Santo Domingo; -SJMT San José; -SMT Santiago, Simferopol, Singapore, Stanley; -TBMT Tbilisi; -TMT Tallinn, Tehran; -WMT Warsaw</small>.</p> + <p> + <small>These abbreviations (for standard/daylight/etc. time) are: + ACST/ACDT Australian Central, + AST/ADT/APT/AWT/ADDT Atlantic, + AEST/AEDT Australian Eastern, + AHST/AHDT Alaska-Hawaii, + AKST/AKDT Alaska, + AWST/AWDT Australian Western, + BST/BDT Bering, + CAT/CAST Central Africa, + CET/CEST/CEMT Central European, + ChST Chamorro, + CST/CDT/CWT/CPT/CDDT Central [North America], + CST/CDT China, + GMT/BST/IST/BDST Greenwich, + EAT East Africa, + EST/EDT/EWT/EPT/EDDT Eastern [North America], + EET/EEST Eastern European, + GST Guam, + HST/HDT Hawaii, + HKT/HKST Hong Kong, + IST India, + IST/GMT Irish, + IST/IDT/IDDT Israel, + JST/JDT Japan, + KST/KDT Korea, + MET/MEST Middle European (a backward-compatibility alias for + Central European), + MSK/MSD Moscow, + MST/MDT/MWT/MPT/MDDT Mountain, + NST/NDT/NWT/NPT/NDDT Newfoundland, + NST/NDT/NWT/NPT Nome, + NZMT/NZST New Zealand through 1945, + NZST/NZDT New Zealand 1946–present, + PKT/PKST Pakistan, + PST/PDT/PWT/PPT/PDDT Pacific, + SAST South Africa, + SST Samoa, + WAT/WAST West Africa, + WET/WEST/WEMT Western European, + WIB Waktu Indonesia Barat, + WIT Waktu Indonesia Timur, + WITA Waktu Indonesia Tengah, + YST/YDT/YWT/YPT/YDDT Yukon</small>. + </p> + </li> + <li> + <p> + For zones whose times are taken from a city's longitude, use the + traditional <var>x</var>MT notation. + The only abbreviation like this in current use is '<abbr>GMT</abbr>'. + The others are for timestamps before 1960, + except that Monrovia Mean Time persisted until 1972. + Typically, numeric abbreviations (e.g., '<code>-</code>004430' for + MMT) would cause trouble here, as the numeric strings would exceed + the POSIX length limit. + </p> -<p><small>A few abbreviations also follow the pattern that -GMT/BST established for time in the UK. They are: + <p> + <small>These abbreviations are: + AMT Amsterdam, Asunción, Athens; + BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels, + Bucharest; + CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; + DMT Dublin/Dunsink; + EMT Easter; + FFMT Fort-de-France; + FMT Funchal; + GMT Greenwich; + HMT Havana, Helsinki, Horta, Howrah; + IMT Irkutsk, Istanbul; + JMT Jerusalem; + KMT Kaunas, Kiev, Kingston; + LMT Lima, Lisbon, local, Luanda; + MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, + Moratuwa, Moscow; + PLMT Phù Liễn; + PMT Paramaribo, Paris, Perm, Pontianak, Prague; + PMMT Port Moresby; + QMT Quito; + RMT Rangoon, Riga, Rome; + SDMT Santo Domingo; + SJMT San José; + SMT Santiago, Simferopol, Singapore, Stanley; + TBMT Tbilisi; + TMT Tallinn, Tehran; + WMT Warsaw</small>. + </p> -CMT/BST for Calamarca Mean Time and Bolivian Summer Time -1890–1932, DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time -1880–1916, MMT/MST/MDST for Moscow 1880–1919, and RMT/LST -for Riga Mean Time and Latvian Summer time 1880–1926. -An extra-special case is SET for Swedish Time (<em>svensk -normaltid</em>) 1879–1899, 3° west of the Stockholm -Observatory.</small></p> + <p> + <small>A few abbreviations also follow the pattern that + <abbr>GMT<abbr>/<abbr>BST</abbr> established for time in the UK. + They are: + CMT/BST for Calamarca Mean Time and Bolivian Summer Time + 1890–1932, + DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time + 1880–1916, + MMT/MST/MDST for Moscow 1880–1919, and + RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926. + An extra-special case is SET for Swedish Time (<em>svensk + normaltid</em>) 1879–1899, 3° west of the Stockholm + Observatory.</small> + </p> </li> <li> - Use 'LMT' for local mean time of locations before the introduction - of standard time; see "<a href="#scope">Scope of the - tz database</a>". + Use '<abbr>LMT</abbr>' for local mean time of locations before the + introduction of standard time; see "<a href="#scope">Scope of the + <code><abbr>tz</abbr></code> database</a>". </li> <li> - If there is no common English abbreviation, use numeric offsets like - <code>-</code>05 and <code>+</code>0830 that are - generated by zic's <code>%z</code> notation. + If there is no common English abbreviation, use numeric offsets like + <code>-</code>05 and <code>+</code>0830 that are generated + by <code>zic</code>'s <code>%z</code> notation. </li> <li> - Use current abbreviations for older timestamps to avoid confusion. - For example, in 1910 a common English abbreviation for UT +01 - in central Europe was 'MEZ' (short for both "Middle European - Zone" and for "Mitteleuropäische Zeit" in German). Nowadays - 'CET' ("Central European Time") is more common in English, and - the database uses 'CET' even for circa-1910 timestamps as this - is less confusing for modern users and avoids the need for - determining when 'CET' supplanted 'MEZ' in common usage. + Use current abbreviations for older timestamps to avoid confusion. + For example, in 1910 a common English abbreviation for time + in central Europe was 'MEZ' (short for both "Middle European + Zone" and for "Mitteleuropäische Zeit" in German). + Nowadays 'CET' ("Central European Time") is more common in + English, and the database uses 'CET' even for circa-1910 + timestamps as this is less confusing for modern users and avoids + the need for determining when 'CET' supplanted 'MEZ' in common + usage. </li> <li> - Use a consistent style in a zone's history. For example, if a zone's - history tends to use numeric abbreviations and a particular - entry could go either way, use a numeric abbreviation. + Use a consistent style in a zone's history. + For example, if a zone's history tends to use numeric + abbreviations and a particular entry could go either way, use a + numeric abbreviation. </li> <li> - Use UT (with time zone abbreviation '<code>-</code>00') for - locations while uninhabited. The leading - '<code>-</code>' is a flag that the time - zone is in some sense undefined; this notation is - derived from Internet RFC 3339. + Use <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> + (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for + locations while uninhabited. + The leading '<code>-</code>' is a flag that the time zone is in + some sense undefined; this notation is derived + from <a href="https://tools.ietf.org/html/rfc3339">Internet + <abbr title="Request For Comments">RFC 3339</a>. </li> </ul> + <p> Application writers should note that these abbreviations are ambiguous in practice: e.g., 'CST' means one thing in China and something else in North America, and 'IST' can refer to time in India, Ireland or -Israel. To avoid ambiguity, use numeric UT offsets like +Israel. +To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like '<code>-</code>0600' instead of time zone abbreviations like 'CST'. </p> - </section> - +</section> - <section> - <h2 id="accuracy">Accuracy of the tz database</h2> +<section> + <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2> <p> -The tz database is not authoritative, and it surely has errors. +The <code><abbr>tz</abbr></code> database is not authoritative, and it +surely has errors. Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>. Users requiring authoritative data should consult national standards bodies and the references cited in the database's comments. </p> <p> -Errors in the tz database arise from many sources: +Errors in the <code><abbr>tz</abbr></code> database arise from many sources: </p> + <ul> <li> - The tz database predicts future timestamps, and current predictions - will be incorrect after future governments change the rules. - For example, if today someone schedules a meeting for 13:00 next - October 1, Casablanca time, and tomorrow Morocco changes its - daylight saving rules, software can mess up after the rule change - if it blithely relies on conversions made before the change. - </li> - <li> - The pre-1970 entries in this database cover only a tiny sliver of how - clocks actually behaved; the vast majority of the necessary - information was lost or never recorded. Thousands more zones would - be needed if the tz database's scope were extended to cover even - just the known or guessed history of standard time; for example, - the current single entry for France would need to split into dozens - of entries, perhaps hundreds. And in most of the world even this - approach would be misleading due to widespread disagreement or - indifference about what times should be observed. In her 2015 book - <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes - "Outside of Europe and North America there was no system of time - zones at all, often not even a stable landscape of mean times, - prior to the middle decades of the twentieth century". See: - Timothy Shenk, <a - href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vaness…">Booked: - A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. - </li> - <li> - Most of the pre-1970 data entries come from unreliable sources, often - astrology books that lack citations and whose compilers evidently - invented entries when the true facts were unknown, without - reporting which entries were known and which were invented. - These books often contradict each other or give implausible entries, - and on the rare occasions when they are checked they are - typically found to be incorrect. - </li> - <li> - For the UK the tz database relies on years of first-class work done by - Joseph Myers and others; see - "<a href="https://www.polyomino.org.uk/british-time/">History of - legal time in Britain</a>". - Other countries are not done nearly as well. - </li> - <li> - Sometimes, different people in the same city would maintain clocks - that differed significantly. Railway time was used by railroad - companies (which did not always agree with each other), - church-clock time was used for birth certificates, etc. - Often this was merely common practice, but sometimes it was set by law. - For example, from 1891 to 1911 the UT offset in France was legally - 0:09:21 outside train stations and 0:04:21 inside. - </li> - <li> - Although a named location in the tz database stands for the - containing region, its pre-1970 data entries are often accurate for - only a small subset of that region. For example, <code>Europe/London</code> - stands for the United Kingdom, but its pre-1847 times are valid - only for locations that have London's exact meridian, and its 1847 - transition to GMT is known to be valid only for the L&NW and the - Caledonian railways. - </li> - <li> - The tz database does not record the earliest time for which a zone's - data entries are thereafter valid for every location in the region. - For example, <code>Europe/London</code> is valid for all locations in its - region after GMT was made the standard time, but the date of - standardization (1880-08-02) is not in the tz database, other than - in commentary. For many zones the earliest time of validity is - unknown. - </li> - <li> - The tz database does not record a region's boundaries, and in many - cases the boundaries are not known. For example, the zone - <code>America/Kentucky/Louisville</code> represents a region around - the city of - Louisville, the boundaries of which are unclear. - </li> - <li> - Changes that are modeled as instantaneous transitions in the tz - database were often spread out over hours, days, or even decades. - </li> - <li> - Even if the time is specified by law, locations sometimes - deliberately flout the law. - </li> - <li> - Early timekeeping practices, even assuming perfect clocks, were - often not specified to the accuracy that the tz database requires. - </li> - <li> - Sometimes historical timekeeping was specified more precisely - than what the tz database can handle. For example, from 1909 to - 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz - database cannot represent the fractional second. - </li> - <li> - Even when all the timestamp transitions recorded by the tz database - are correct, the tz rules that generate them may not faithfully - reflect the historical rules. For example, from 1922 until World - War II the UK moved clocks forward the day following the third - Saturday in April unless that was Easter, in which case it moved - clocks forward the previous Sunday. Because the tz database has no - way to specify Easter, these exceptional years are entered as - separate tz Rule lines, even though the legal rules did not change. - </li> - <li> - The tz database models pre-standard time using the proleptic Gregorian - calendar and local mean time (LMT), but many people used other - calendars and other timescales. For example, the Roman Empire used - the Julian calendar, and had 12 varying-length daytime hours with a - non-hour-based system at night. - </li> - <li> - Early clocks were less reliable, and data entries do not represent - clock error. - </li> - <li> - The tz database assumes Universal Time (UT) as an origin, even - though UT is not standardized for older timestamps. In the tz - database commentary, UT denotes a family of time standards that - includes Coordinated Universal Time (UTC) along with other variants - such as UT1 and GMT, with days starting at midnight. Although UT - equals UTC for modern timestamps, UTC was not defined until 1960, - so commentary uses the more-general abbreviation UT for timestamps - that might predate 1960. Since UT, UT1, etc. disagree slightly, - and since pre-1972 UTC seconds varied in length, interpretation of - older timestamps can be problematic when subsecond accuracy is - needed. - </li> - <li> - Civil time was not based on atomic time before 1972, and we don't - know the history of earth's rotation accurately enough to map SI - seconds to historical solar time to more than about one-hour - accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY. - <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement - of the Earth's rotation: 720 BC to AD 2015</a>. - <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. - Also see: Espenak F. <a - href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty - in Delta T (ΔT)</a>. - </li> - <li> - The relationship between POSIX time (that is, UTC but ignoring leap - seconds) and UTC is not agreed upon after 1972. Although the POSIX - clock officially stops during an inserted leap second, at least one - proposed standard has it jumping back a second instead; and in - practice POSIX clocks more typically either progress glacially during - a leap second, or are slightly slowed while near a leap second. - </li> - <li> - The tz database does not represent how uncertain its information is. - Ideally it would contain information about when data entries are - incomplete or dicey. Partial temporal knowledge is a field of - active research, though, and it's not clear how to apply it here. + The <code><abbr>tz</abbr></code> database predicts future + timestamps, and current predictions + will be incorrect after future governments change the rules. + For example, if today someone schedules a meeting for 13:00 next + October 1, Casablanca time, and tomorrow Morocco changes its + daylight saving rules, software can mess up after the rule change + if it blithely relies on conversions made before the change. + </li> + <li> + The pre-1970 entries in this database cover only a tiny sliver of how + clocks actually behaved; the vast majority of the necessary + information was lost or never recorded. + Thousands more zones would be needed if + the <code><abbr>tz</abbr></code> database's scope were extended to + cover even just the known or guessed history of standard time; for + example, the current single entry for France would need to split + into dozens of entries, perhaps hundreds. + And in most of the world even this approach would be misleading + due to widespread disagreement or indifference about what times + should be observed. + In her 2015 book + <cite><a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The + Global Transformation of Time, 1870–1950</a></cite>, + Vanessa Ogle writes + "Outside of Europe and North America there was no system of time + zones at all, often not even a stable landscape of mean times, + prior to the middle decades of the twentieth century". + See: Timothy Shenk, <a +href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vaness…">Booked: + A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. + </li> + <li> + Most of the pre-1970 data entries come from unreliable sources, often + astrology books that lack citations and whose compilers evidently + invented entries when the true facts were unknown, without + reporting which entries were known and which were invented. + These books often contradict each other or give implausible entries, + and on the rare occasions when they are checked they are + typically found to be incorrect. + </li> + <li> + For the UK the <code><abbr>tz</abbr></code> database relies on + years of first-class work done by + Joseph Myers and others; see + "<a href="https://www.polyomino.org.uk/british-time/">History of + legal time in Britain</a>". + Other countries are not done nearly as well. + </li> + <li> + Sometimes, different people in the same city would maintain clocks + that differed significantly. + Railway time was used by railroad companies (which did not always + agree with each other), church-clock time was used for birth + certificates, etc. + Often this was merely common practice, but sometimes it was set by law. + For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France + was legally <abbr>UT</abbr> +00:09:21 outside train stations and + <abbr>UT</abbr> +00:04:21 inside. + </li> + <li> + Although a named location in the <code><abbr>tz</abbr></code> + database stands for the containing region, its pre-1970 data + entries are often accurate for only a small subset of that region. + For example, <code>Europe/London</code> stands for the United + Kingdom, but its pre-1847 times are valid only for locations that + have London's exact meridian, and its 1847 transition + to <abbr>GMT</abbr> is known to be valid only for the L&NW and + the Caledonian railways. + </li> + <li> + The <code><abbr>tz</abbr></code> database does not record the + earliest time for which a zone's + data entries are thereafter valid for every location in the region. + For example, <code>Europe/London</code> is valid for all locations + in its region after <abbr>GMT</abbr> was made the standard time, + but the date of standardization (1880-08-02) is not in the + <code><abbr>tz</abbr></code> database, other than in commentary. + For many zones the earliest time of validity is unknown. + </li> + <li> + The <code><abbr>tz</abbr></code> database does not record a + region's boundaries, and in many cases the boundaries are not known. + For example, the zone + <code>America/Kentucky/Louisville</code> represents a region + around the city of Louisville, the boundaries of which are + unclear. + </li> + <li> + Changes that are modeled as instantaneous transitions in the + <code><abbr>tz</abbr></code> + database were often spread out over hours, days, or even decades. + </li> + <li> + Even if the time is specified by law, locations sometimes + deliberately flout the law. + </li> + <li> + Early timekeeping practices, even assuming perfect clocks, were + often not specified to the accuracy that the + <code><abbr>tz</abbr></code> database requires. + </li> + <li> + Sometimes historical timekeeping was specified more precisely + than what the <code><abbr>tz</abbr></code> database can handle. + For example, from 1909 to 1937 <a + href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" + hreflang="nl">Netherlands clocks</a> were legally <abbr>UT</abbr> + +00:19:32.13, but the <code><abbr>tz</abbr></code> + database cannot represent the fractional second. + </li> + <li> + Even when all the timestamp transitions recorded by the + <code><abbr>tz</abbr></code> database are correct, the + <code><abbr>tz</abbr></code> rules that generate them may not + faithfully reflect the historical rules. + For example, from 1922 until World War II the UK moved clocks + forward the day following the third Saturday in April unless that + was Easter, in which case it moved clocks forward the previous + Sunday. + Because the <code><abbr>tz</abbr></code> database has no + way to specify Easter, these exceptional years are entered as + separate <code><abbr>tz</abbr> Rule</code> lines, even though the + legal rules did not change. + </li> + <li> + The <code><abbr>tz</abbr></code> database models pre-standard time + using the <a + href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic + Gregorian calendar</a> and local mean time, but many people used + other calendars and other timescales. + For example, the Roman Empire used + the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian + calendar</a>, + and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman + timekeeping</a> had twelve varying-length daytime hours with a + non-hour-based system at night. + </li> + <li> + Early clocks were less reliable, and data entries do not represent + clock error. + </li> + <li> + The <code><abbr>tz</abbr></code> database assumes Universal Time + (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not + standardized for older timestamps. + In the <code><abbr>tz</abbr></code> database commentary, + <abbr>UT</abbr> denotes a family of time standards that includes + Coordinated Universal Time (<abbr>UTC</abbr>) along with other + variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>, + with days starting at midnight. + Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern + timestamps, <abbr>UTC</abbr> was not defined until 1960, so + commentary uses the more-general abbreviation <abbr>UT</abbr> for + timestamps that might predate 1960. + Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly, + and since pre-1972 <abbr>UTC</abbr> seconds varied in length, + interpretation of older timestamps can be problematic when + subsecond accuracy is needed. + </li> + <li> + Civil time was not based on atomic time before 1972, and we don't + know the history of + <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's + rotation</a> accurately enough to map <a + href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr + title="International System of Units">SI</abbr></a> seconds to + historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a> + to more than about one-hour accuracy. + See: Stephenson FR, Morrison LV, Hohenkerk CY. + <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement of + the Earth's rotation: 720 BC to AD 2015</a>. + <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. + Also see: Espenak F. <a + href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty + in Delta T (ΔT)</a>. + </li> + <li> + The relationship between POSIX time (that is, <abbr>UTC</abbr> but + ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap + seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972. + Although the POSIX + clock officially stops during an inserted leap second, at least one + proposed standard has it jumping back a second instead; and in + practice POSIX clocks more typically either progress glacially during + a leap second, or are slightly slowed while near a leap second. + </li> + <li> + The <code><abbr>tz</abbr></code> database does not represent how + uncertain its information is. + Ideally it would contain information about when data entries are + incomplete or dicey. + Partial temporal knowledge is a field of active research, though, + and it's not clear how to apply it here. </li> </ul> + <p> -In short, many, perhaps most, of the tz database's pre-1970 and future -timestamps are either wrong or misleading. Any attempt to pass the -tz database off as the definition of time should be unacceptable to -anybody who cares about the facts. In particular, the tz database's -LMT offsets should not be considered meaningful, and should not prompt -creation of zones merely because two locations differ in LMT or -transitioned to standard time at different dates. +In short, many, perhaps most, of the <code><abbr>tz</abbr></code> +database's pre-1970 and future timestamps are either wrong or +misleading. +Any attempt to pass the +<code><abbr>tz</abbr></code> database off as the definition of time +should be unacceptable to anybody who cares about the facts. +In particular, the <code><abbr>tz</abbr></code> database's +<abbr>LMT</abbr> offsets should not be considered meaningful, and +should not prompt creation of zones merely because two locations +differ in <abbr>LMT</abbr> or transitioned to standard time at +different dates. </p> - </section> - +</section> - <section> - <h2 id="functions">Time and date functions</h2> +<section> + <h2 id="functions">Time and date functions</h2> <p> -The tz code contains time and date functions that are upwards -compatible with those of POSIX. +The <code><abbr>tz</abbr></code> code contains time and date functions +that are upwards compatible with those of POSIX. Code compatible with this package is already -<a href="tz-link.html#tzdb">part of many platforms</a>, -where the primary use of this package -is to update obsolete time zone rule tables. +<a href="tz-link.html#tzdb">part of many platforms</a>, where the +primary use of this package is to update obsolete time zone rule +tables. To do this, you may need to compile the time zone compiler -'<code>zic</code>' supplied with this package instead of using -the system '<code>zic</code>', since the format -of <code>zic</code>'s input is occasionally extended, and a -platform may still be shipping an older <code>zic</code>. +'<code>zic</code>' supplied with this package instead of using the +system '<code>zic</code>', since the format of <code>zic</code>'s +input is occasionally extended, and a platform may still be shipping +an older <code>zic</code>. </p> <h3 id="POSIX">POSIX properties and limitations</h3> <ul> <li> <p> - In POSIX, time display in a process is controlled by the - environment variable TZ. Unfortunately, the POSIX TZ string takes - a form that is hard to describe and is error-prone in practice. - Also, POSIX TZ strings can't deal with other (for example, Iranian) - daylight saving time rules, or situations where more than two - time zone abbreviations are used in an area. + In POSIX, time display in a process is controlled by the + environment variable <code>TZ</code>. + Unfortunately, the POSIX + <code>TZ</code> string takes a form that is hard to describe and + is error-prone in practice. + Also, POSIX <code>TZ</code> strings can't deal with daylight + saving time rules not based on the Gregorian calendar (as in + Iran), or with situations where more than two time zone + abbreviations or <abbr>UT</abbr> offsets are used in an area. </p> + <p> - The POSIX TZ string takes the following form: + The POSIX <code>TZ</code> string takes the following form: </p> + <p> - <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]] + <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]] </p> + <p> - where: + where: + </p> + <dl> <dt><var>std</var> and <var>dst</var></dt><dd> - are 3 or more characters specifying the standard - and daylight saving time (DST) zone names. - Starting with POSIX.1-2001, <var>std</var> - and <var>dst</var> may also be - in a quoted form like '<code><+09></code>'; this allows - "<code>+</code>" and "<code>-</code>" in the names. + are 3 or more characters specifying the standard + and daylight saving time (<abbr>DST</abbr>) zone names. + Starting with POSIX.1-2001, <var>std</var> and <var>dst</var> + may also be in a quoted form like '<code><+09></code>'; + this allows "<code>+</code>" and "<code>-</code>" in the names. </dd> <dt><var>offset</var></dt><dd> - is of the form - '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' - and specifies the offset west of UT. '<var>hh</var>' - may be a single digit; 0≤<var>hh</var>≤24. - The default DST offset is one hour ahead of standard time. + is of the form + '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' + and specifies the offset west of <abbr>UT</abbr>. + '<var>hh</var>' may be a single digit; + 0≤<var>hh</var>≤24. + The default <abbr>DST</abbr> offset is one hour ahead of + standard time. </dd> <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> - specifies the beginning and end of DST. If this is absent, - the system supplies its own rules for DST, and these can - differ from year to year; typically US DST rules are used. + specifies the beginning and end of <abbr>DST</abbr>. + If this is absent, the system supplies its own rules + for <abbr>DST</abbr>, and these can differ from year to year; + typically <abbr>US</abbr> <abbr>DST</abbr> rules are used. </dd> <dt><var>time</var></dt><dd> - takes the form - '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' - and defaults to 02:00. - This is the same format as the offset, except that a - leading '<code>+</code>' or '<code>-</code>' is not allowed. + takes the form + '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' + and defaults to 02:00. + This is the same format as the offset, except that a + leading '<code>+</code>' or '<code>-</code>' is not allowed. </dd> <dt><var>date</var></dt><dd> - takes one of the following forms: + takes one of the following forms: <dl> <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> - origin-1 day number not counting February 29 - </dd> + origin-1 day number not counting February 29 + </dd> <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> - origin-0 day number counting February 29 if present - </dd> - <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 1≤<var>m</var>≤12)</dt><dd> - for the <var>d</var>th day of - week <var>n</var> of month <var>m</var> of the - year, where week 1 is the first week in which - day <var>d</var> appears, and '<code>5</code>' - stands for the last week in which - day <var>d</var> appears - (which may be either the 4th or 5th week). - Typically, this is the only useful form; - the <var>n</var> - and <code>J</code><var>n</var> forms are - rarely used. + origin-0 day number counting February 29 if present </dd> -</dl> -</dd> -</dl> - Here is an example POSIX TZ string for New Zealand after 2007. - It says that standard time (NZST) is 12 hours ahead of UT, - and that daylight saving time (NZDT) is observed from September's - last Sunday at 02:00 until April's first Sunday at 03:00: + <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> + (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, + 1≤<var>m</var>≤12)</dt><dd> + for the <var>d</var>th day of week <var>n</var> of + month <var>m</var> of the year, where week 1 is the first + week in which day <var>d</var> appears, and + '<code>5</code>' stands for the last week in which + day <var>d</var> appears (which may be either the 4th or + 5th week). + Typically, this is the only useful form; the <var>n</var> + and <code>J</code><var>n</var> forms are rarely used. + </dd> + </dl> + </dd> + </dl> - <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> + <p> + Here is an example POSIX <code>TZ</code> string for New + Zealand after 2007. + It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead + of <abbr>UT</abbr>, and that daylight saving time + (<abbr>NZDT</abbr>) is observed from September's last Sunday at + 02:00 until April's first Sunday at 03:00: + </p> - This POSIX TZ string is hard to remember, and mishandles some - timestamps before 2008. With this package you can use this - instead: + <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> - <pre><code>TZ='Pacific/Auckland'</code></pre> + <p> + This POSIX <code>TZ</code> string is hard to remember, and + mishandles some timestamps before 2008. + With this package you can use this instead: + </p> + + <pre><code>TZ='Pacific/Auckland'</code></pre> + </li> + <li> + POSIX does not define the exact meaning of <code>TZ</code> values like + "<code>EST5EDT</code>". + Typically the current <abbr>US</abbr> <abbr>DST</abbr> rules + are used to interpret such values, but this means that the + <abbr>US</abbr> <abbr>DST</abbr> rules are compiled into each + program that does time conversion. + This means that when + <abbr>US</abbr> time conversion rules change (as in the United + States in 1987), all programs that do time conversion must be + recompiled to ensure proper results. + </li> + <li> + The <code>TZ</code> environment variable is process-global, which + makes it hard to write efficient, thread-safe applications that + need access to multiple time zones. + </li> + <li> + In POSIX, there's no tamper-proof way for a process to learn the + system's best idea of local wall clock. + (This is important for applications that an administrator wants + used only at certain times – without regard to whether the + user has fiddled the + <code>TZ</code> environment variable. + While an administrator can "do everything in <abbr>UT</abbr>" to + get around the problem, doing so is inconvenient and precludes + handling daylight saving time shifts - as might be required to + limit phone calls to off-peak hours.) + </li> + <li> + POSIX provides no convenient and efficient way to determine + the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary + timestamps, particularly for time zone settings that do not fit + into the POSIX model. + </li> + <li> + POSIX requires that systems ignore leap seconds. + </li> + <li> + The <code><abbr>tz</abbr></code> code attempts to support all the + <code>time_t</code> implementations allowed by POSIX. + The <code>time_t</code> type represents a nonnegative count of seconds + since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds. + In practice, <code>time_t</code> is usually a signed 64- or 32-bit + integer; 32-bit signed <code>time_t</code> values stop working after + 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these + days typically use a signed 64-bit integer. + Unsigned 32-bit integers are used on one or two platforms, and 36-bit + and 40-bit integers are also used occasionally. + Although earlier POSIX versions allowed <code>time_t</code> to be a + floating-point type, this was not supported by any practical systems, + and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both + require <code>time_t</code> to be an integer type. </li> +</ul> + +<h3 id="POSIX-extensions">Extensions to POSIX in the +<code><abbr>tz</abbr></code> code</h3> +<ul> <li> - POSIX does not define the exact meaning of TZ values like - "<code>EST5EDT</code>". - Typically the current US DST rules are used to interpret such values, - but this means that the US DST rules are compiled into each program - that does time conversion. This means that when US time conversion - rules change (as in the United States in 1987), all programs that - do time conversion must be recompiled to ensure proper results. + <p> + The <code>TZ</code> environment variable is used in generating + the name of a file from which time zone information is read + (or is interpreted à la POSIX); <code>TZ</code> is no longer + constrained to be a three-letter time zone + name followed by a number of hours and an optional three-letter + daylight time zone name. + The daylight saving time rules to be used for a particular time + zone are encoded in the time zone file; the format of the file + allows U.S., Australian, and other rules to be encoded, and + allows for situations where more than two time zone + abbreviations are used. + </p> + <p> + It was recognized that allowing the <code>TZ</code> environment + variable to take on values such as '<code>America/New_York</code>' + might cause "old" programs (that expect <code>TZ</code> to have a + certain form) to operate incorrectly; consideration was given to using + some other environment variable (for example, <code>TIMEZONE</code>) + to hold the string used to generate the time zone information file name. + In the end, however, it was decided to continue using + <code>TZ</code>: it is widely used for time zone purposes; + separately maintaining both <code>TZ</code> + and <code>TIMEZONE</code> seemed a nuisance; and systems where + "new" forms of <code>TZ</code> might cause problems can simply + use <code>TZ</code> values such as "<code>EST5EDT</code>" which + can be used both by "new" programs (à la POSIX) and "old" + programs (as zone names and offsets). + </p> </li> <li> - The TZ environment variable is process-global, which makes it hard - to write efficient, thread-safe applications that need access - to multiple time zones. + The code supports platforms with a <abbr>UT</abbr> offset member + in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>. </li> <li> - In POSIX, there's no tamper-proof way for a process to learn the - system's best idea of local wall clock. (This is important for - applications that an administrator wants used only at certain - times – - without regard to whether the user has fiddled the TZ environment - variable. While an administrator can "do everything in UT" to get - around the problem, doing so is inconvenient and precludes handling - daylight saving time shifts - as might be required to limit phone - calls to off-peak hours.) + The code supports platforms with a time zone abbreviation member in + <code>struct tm</code>, e.g., <code>tm_zone</code>. </li> <li> - POSIX provides no convenient and efficient way to determine the UT - offset and time zone abbreviation of arbitrary timestamps, - particularly for time zone settings that do not fit into the - POSIX model. + Functions <code>tzalloc</code>, <code>tzfree</code>, + <code>localtime_rz</code>, and <code>mktime_z</code> for + more-efficient thread-safe applications that need to use multiple + time zones. + The <code>tzalloc</code> and <code>tzfree</code> functions + allocate and free objects of type <code>timezone_t</code>, + and <code>localtime_rz</code> and <code>mktime_z</code> are + like <code>localtime_r</code> and <code>mktime</code> with an + extra <code>timezone_t</code> argument. + The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>. </li> <li> - POSIX requires that systems ignore leap seconds. + A function <code>tzsetwall</code> has been added to arrange for the + system's best approximation to local wall clock time to be delivered + by subsequent calls to <code>localtime</code>. + Source code for portable applications that "must" run on local wall + clock time should call <code>tzsetwall</code>; + if such code is moved to "old" systems that don't + provide <code>tzsetwall</code>, you won't be able to generate an + executable program. + (These time zone functions also arrange for local wall clock time to + be used if <code>tzset</code> is called – directly or + indirectly – and there's no <code>TZ</code> environment + variable; portable applications should not, however, rely on this + behavior since it's not the way SVR2 systems behave.) </li> <li> - The tz code attempts to support all the <code>time_t</code> - implementations allowed by POSIX. The <code>time_t</code> - type represents a nonnegative count of - seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. - In practice, <code>time_t</code> is usually a signed 64- or - 32-bit integer; 32-bit signed <code>time_t</code> values stop - working after 2038-01-19 03:14:07 UTC, so - new implementations these days typically use a signed 64-bit integer. - Unsigned 32-bit integers are used on one or two platforms, - and 36-bit and 40-bit integers are also used occasionally. - Although earlier POSIX versions allowed <code>time_t</code> to be a - floating-point type, this was not supported by any practical - systems, and POSIX.1-2013 and the tz code both - require <code>time_t</code> - to be an integer type. + Negative <code>time_t</code> values are supported, on systems + where <code>time_t</code> is signed. </li> -</ul> - -<h3 id="POSIX-extensions">Extensions to POSIX in the tz code</h3> -<ul> <li> - <p> - The TZ environment variable is used in generating the name of a file - from which time zone information is read (or is interpreted a la - POSIX); TZ is no longer constrained to be a three-letter time zone - name followed by a number of hours and an optional three-letter - daylight time zone name. The daylight saving time rules to be used - for a particular time zone are encoded in the time zone file; - the format of the file allows U.S., Australian, and other rules to be - encoded, and allows for situations where more than two time zone - abbreviations are used. - </p> - <p> - It was recognized that allowing the TZ environment variable to - take on values such as '<code>America/New_York</code>' might - cause "old" programs - (that expect TZ to have a certain form) to operate incorrectly; - consideration was given to using some other environment variable - (for example, TIMEZONE) to hold the string used to generate the - time zone information file name. In the end, however, it was decided - to continue using TZ: it is widely used for time zone purposes; - separately maintaining both TZ and TIMEZONE seemed a nuisance; - and systems where "new" forms of TZ might cause problems can simply - use TZ values such as "<code>EST5EDT</code>" which can be used both by - "new" programs (a la POSIX) and "old" programs (as zone names and - offsets). - </p> -</li> -<li> - The code supports platforms with a UT offset member - in <code>struct tm</code>, - e.g., <code>tm_gmtoff</code>. -</li> -<li> - The code supports platforms with a time zone abbreviation member in - <code>struct tm</code>, e.g., <code>tm_zone</code>. -</li> -<li> - Functions <code>tzalloc</code>, <code>tzfree</code>, - <code>localtime_rz</code>, and <code>mktime_z</code> for - more-efficient thread-safe applications that need to use - multiple time zones. The <code>tzalloc</code> - and <code>tzfree</code> functions allocate and free objects of - type <code>timezone_t</code>, and <code>localtime_rz</code> - and <code>mktime_z</code> are like <code>localtime_r</code> - and <code>mktime</code> with an extra - <code>timezone_t</code> argument. The functions were inspired - by NetBSD. -</li> -<li> - A function <code>tzsetwall</code> has been added to arrange - for the system's - best approximation to local wall clock time to be delivered by - subsequent calls to <code>localtime</code>. Source code for portable - applications that "must" run on local wall clock time should call - <code>tzsetwall</code>; if such code is moved to "old" systems that don't - provide tzsetwall, you won't be able to generate an executable program. - (These time zone functions also arrange for local wall clock time to be - used if tzset is called – directly or indirectly – - and there's no TZ - environment variable; portable applications should not, however, rely - on this behavior since it's not the way SVR2 systems behave.) -</li> -<li> - Negative <code>time_t</code> values are supported, on systems - where <code>time_t</code> is signed. -</li> -<li> - These functions can account for leap seconds, thanks to Bradley White. -</li> + These functions can account for leap seconds, thanks to Bradley White. + </li> </ul> <h3 id="vestigial">POSIX features no longer needed</h3> <p> -POSIX and ISO C define some APIs that are vestigial: they are not -needed, and are relics of a too-simple model that does not suffice to -handle many real-world timestamps. Although the tz code supports these -vestigial APIs for backwards compatibility, they should be avoided in -portable applications. The vestigial APIs are: +POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a> +define some <a href="https://en.wikipedia.org/wiki/API"><abbr +title="application programming interface">API</abbr>s</a> that are vestigial: +they are not needed, and are relics of a too-simple model that does +not suffice to handle many real-world timestamps. +Although the <code><abbr>tz</abbr></code> code supports these +vestigial <abbr>API</abbr>s for backwards compatibility, they should +be avoided in portable applications. +The vestigial <abbr>API</abbr>s are: </p> <ul> <li> The POSIX <code>tzname</code> variable does not suffice and is no - longer needed. To get a timestamp's time zone abbreviation, - consult the <code>tm_zone</code> member if available; otherwise, + longer needed. + To get a timestamp's time zone abbreviation, consult + the <code>tm_zone</code> member if available; otherwise, use <code>strftime</code>'s <code>"%Z"</code> conversion specification. </li> <li> The POSIX <code>daylight</code> and <code>timezone</code> - variables do not suffice and are no longer needed. To get a - timestamp's UT offset, consult the <code>tm_gmtoff</code> member - if available; otherwise, subtract values returned - by <code>localtime</code> and <code>gmtime</code> using the rules - of the Gregorian calendar, or - use <code>strftime</code>'s <code>"%z"</code> conversion + variables do not suffice and are no longer needed. + To get a timestamp's <abbr>UT</abbr> offset, consult + the <code>tm_gmtoff</code> member if available; otherwise, + subtract values returned by <code>localtime</code> + and <code>gmtime</code> using the rules of the Gregorian calendar, + or use <code>strftime</code>'s <code>"%z"</code> conversion specification if a string like <samp>"+0900"</samp> suffices. </li> <li> The <code>tm_isdst</code> member is almost never needed and most of its uses should be discouraged in favor of the abovementioned - APIs. Although it can still be used in arguments to - <code>mktime</code> to disambiguate timestamps near a DST - transition when the clock jumps back, this disambguation does not - work when standard time itself jumps back, which can occur when a - location changes to a time zone with a lesser UT offset. + <abbr>API</abbr>s. + Although it can still be used in arguments to + <code>mktime</code> to disambiguate timestamps near + a <abbr>DST</abbr> transition when the clock jumps back, this + disambguation does not work when standard time itself jumps back, + which can occur when a location changes to a time zone with a + lesser <abbr>UT</abbr> offset. </li> </ul> <h3 id="other-portability">Other portability notes</h3> <ul> <li> - The UNIX Version 7 <code>timezone</code> function is not - present in this package; - it's impossible to reliably map timezone's arguments (a "minutes west - of GMT" value and a "daylight saving time in effect" flag) to a - time zone abbreviation, and we refuse to guess. - Programs that in the past used the timezone function may now examine - <code>localtime(&clock)->tm_zone</code> - (if <code>TM_ZONE</code> is defined) or - <code>tzname[localtime(&clock)->tm_isdst]</code> - (if <code>HAVE_TZNAME</code> is defined) - to learn the correct time zone abbreviation to use. - </li> - <li> - The 4.2BSD <code>gettimeofday</code> function is not used in - this package. - This formerly let users obtain the current UTC offset and DST flag, - but this functionality was removed in later versions of BSD. - </li> - <li> - In SVR2, time conversion fails for near-minimum or near-maximum - <code>time_t</code> values when doing conversions for places - that don't use UT. - This package takes care to do these conversions correctly. - A comment in the source code tells how to get compatibly wrong - results. - </li> -<li> -The functions that are conditionally compiled -if <code>STD_INSPIRED</code> is defined -should, at this point, be looked on primarily as food for thought. They are -not in any sense "standard compatible" – some are not, in fact, -specified in <em>any</em> standard. They do, however, represent responses of -various authors to -standardization proposals. -</li> -<li> -Other time conversion proposals, in particular the one developed by folks at -Hewlett Packard, offer a wider selection of functions that provide capabilities -beyond those provided here. The absence of such functions from this package -is not meant to discourage the development, standardization, or use of such -functions. Rather, their absence reflects the decision to make this package -contain valid extensions to POSIX, to ensure its broad acceptability. If -more powerful time conversion functions can be standardized, so much the -better. -</li> + The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition + UNIX</a> <code>timezone</code> function is not present in this + package; it's impossible to reliably map <code>timezone</code>'s + arguments (a "minutes west of <abbr>GMT</abbr>" value and a + "daylight saving time in effect" flag) to a time zone + abbreviation, and we refuse to guess. + Programs that in the past used the <code>timezone</code> function + may now examine <code>localtime(&clock)->tm_zone</code> + (if <code>TM_ZONE</code> is defined) or + <code>tzname[localtime(&clock)->tm_isdst]</code> + (if <code>HAVE_TZNAME</code> is defined) to learn the correct time + zone abbreviation to use. + </li> + <li> + The <abbr>4.2BSD</abbr> <code>gettimeofday</code> function is not + used in this package. + This formerly let users obtain the current <abbr>UTC</abbr> offset + and <abbr>DST</abbr> flag, but this functionality was removed in + later versions of <abbr>BSD</abbr>. + </li> + <li> + In <abbr>SVR2</abbr>, time conversion fails for near-minimum or + near-maximum <code>time_t</code> values when doing conversions + for places that don't use <abbr>UT</abbr>. + This package takes care to do these conversions correctly. + A comment in the source code tells how to get compatibly wrong + results. + </li> + <li> + The functions that are conditionally compiled + if <code>STD_INSPIRED</code> is defined should, at this point, be + looked on primarily as food for thought. + They are not in any sense "standard compatible" – some are + not, in fact, specified in <em>any</em> standard. + They do, however, represent responses of various authors to + standardization proposals. + </li> + <li> + Other time conversion proposals, in particular the one developed + by folks at Hewlett Packard, offer a wider selection of functions + that provide capabilities beyond those provided here. + The absence of such functions from this package is not meant to + discourage the development, standardization, or use of such + functions. + Rather, their absence reflects the decision to make this package + contain valid extensions to POSIX, to ensure its broad + acceptability. + If more powerful time conversion functions can be standardized, so + much the better. + </li> </ul> - </section> - +</section> - <section> - <h2 id="stability">Interface stability</h2> +<section> + <h2 id="stability">Interface stability</h2> <p> -The tz code and data supply the following interfaces: +The <code><abbr>tz</abbr></code> code and data supply the following interfaces: </p> + <ul> <li> - A set of zone names as per "<a href="#naming">Names of time zone - rules</a>" above. + A set of zone names as per "<a href="#naming">Names of time zone + rules</a>" above. </li> <li> - Library functions described in "<a href="#functions">Time and date - functions</a>" above. + Library functions described in "<a href="#functions">Time and date + functions</a>" above. </li> <li> - The programs <code>tzselect</code>, <code>zdump</code>, - and <code>zic</code>, documented in their man pages. + The programs <code>tzselect</code>, <code>zdump</code>, + and <code>zic</code>, documented in their man pages. </li> <li> - The format of <code>zic</code> input files, documented in - the <code>zic</code> man page. + The format of <code>zic</code> input files, documented in + the <code>zic</code> man page. </li> <li> - The format of <code>zic</code> output files, documented in - the <code>tzfile</code> man page. + The format of <code>zic</code> output files, documented in + the <code>tzfile</code> man page. </li> <li> - The format of zone table files, documented in <code>zone1970.tab</code>. + The format of zone table files, documented in <code>zone1970.tab</code>. </li> <li> - The format of the country code file, documented in <code>iso3166.tab</code>. + The format of the country code file, documented in <code>iso3166.tab</code>. </li> <li> - The version number of the code and data, as the first line of - the text file '<code>version</code>' in each release. + The version number of the code and data, as the first line of + the text file '<code>version</code>' in each release. </li> </ul> + <p> Interface changes in a release attempt to preserve compatibility with -recent releases. For example, tz data files typically do not rely on -recently-added <code>zic</code> features, so that users can run -older <code>zic</code> versions to process newer data -files. <a href="tz-link.html">Sources for time zone and daylight -saving time data</a> describes how -releases are tagged and distributed. +recent releases. +For example, <code><abbr>tz</abbr></code> data files typically do not +rely on recently-added <code>zic</code> features, so that users can +run older <code>zic</code> versions to process newer data files. +<a href="tz-link.html#download">Downloading +the <code><abbr>tz</abbr></code> database</a> describes how releases +are tagged and distributed. </p> <p> -Interfaces not listed above are less stable. For example, users -should not rely on particular UT offsets or abbreviations for -timestamps, as data entries are often based on guesswork and these -guesses may be corrected or improved. +Interfaces not listed above are less stable. +For example, users should not rely on particular <abbr>UT</abbr> +offsets or abbreviations for timestamps, as data entries are often +based on guesswork and these guesses may be corrected or improved. </p> - </section> - +</section> - <section> - <h2 id="calendar">Calendrical issues</h2> +<section> + <h2 id="calendar">Calendrical issues</h2> <p> Calendrical issues are a bit out of scope for a time zone database, but they indicate the sort of problems that we would run into if we -extended the time zone database further into the past. An excellent -resource in this area is Nachum Dershowitz and Edward M. Reingold, -<cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical +extended the time zone database further into the past. +An excellent resource in this area is Nachum Dershowitz and Edward M. +Reingold, <cite><a +href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical Calculations: Third Edition</a></cite>, Cambridge University Press (2008). Other information and sources are given in the file '<samp>calendars</samp>' -in the tz distribution. They sometimes disagree. +in the <code><abbr>tz</abbr></code> distribution. +They sometimes disagree. </p> - </section> +</section> - - <section> - <h2 id="planets">Time and time zones on other planets</h2> +<section> + <h2 id="planets">Time and time zones on other planets</h2> <p> -Some people's work schedules use Mars time. Jet Propulsion Laboratory -(JPL) coordinators have kept Mars time on and off at least since 1997 -for the Mars Pathfinder mission. Some of their family members have -also adapted to Mars time. Dozens of special Mars watches were built -for JPL workers who kept Mars time during the Mars Exploration -Rovers mission (2004). These timepieces look like normal Seikos and -Citizens but use Mars seconds rather than terrestrial seconds. +Some people's work schedules +use <a href="https://en.wikipedia.org/wiki/Timekeeping on Mars">Mars time</a>. +Jet Propulsion Laboratory (JPL) coordinators have kept Mars time on +and off at least since 1997 for the +<a href="https://en.wikipedia.org/wiki/Mars_Pathfinder#End_of_mission">Mars +Pathfinder</a> mission. +Some of their family members have also adapted to Mars time. +Dozens of special Mars watches were built for JPL workers who kept +Mars time during the Mars Exploration Rovers mission (2004). +These timepieces look like normal Seikos and Citizens but use Mars +seconds rather than terrestrial seconds. </p> <p> A Mars solar day is called a "sol" and has a mean period equal to -about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is -divided into a conventional 24-hour clock, so each Mars second equals -about 1.02749125 terrestrial seconds. +about 24 hours 39 minutes 35.244 seconds in terrestrial time. +It is divided into a conventional 24-hour clock, so each Mars second +equals about 1.02749125 terrestrial seconds. </p> <p> -The prime meridian of Mars goes through the center of the crater -Airy-0, named in honor of the British astronomer who built the -Greenwich telescope that defines Earth's prime meridian. Mean solar -time on the Mars prime meridian is called Mars Coordinated Time (MTC). -</p> +The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime +meridian</a> of Mars goes through the center of the crater +<a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in +honor of the British astronomer who built the Greenwich telescope that +defines Earth's prime meridian. +Mean solar time on the Mars prime meridian is +called <a href="https://en.wikipedia.org/wiki/Mars_Coordinated_Time">Mars +Coordinated Time (<abbr>MTC</abbr>)</a>. +</p> <p> Each landed mission on Mars has adopted a different reference for solar time keeping, so there is no real standard for Mars time zones. -For example, the Mars Exploration Rover project (2004) defined two -time zones "Local Solar Time A" and "Local Solar Time B" for its two -missions, each zone designed so that its time equals local true solar -time at approximately the middle of the nominal mission. Such a "time -zone" is not particularly suited for any application other than the -mission itself. +For example, the +<a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars +Exploration Rover</a> project (2004) defined two time zones "Local +Solar Time A" and "Local Solar Time B" for its two missions, each zone +designed so that its time equals local true solar time at +approximately the middle of the nominal mission. +Such a "time zone" is not particularly suited for any application +other than the mission itself. </p> <p> Many calendars have been proposed for Mars, but none have achieved -wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a +wide acceptance. +Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a sequential count of Mars solar days elapsed since about 1873-12-29 -12:00 GMT. +12:00 <abbr>GMT</abbr>. </p> <p> In our solar system, Mars is the planet with time and calendar most -like Earth's. On other planets, Sun-based time and calendars would -work quite differently. For example, although Mercury's sidereal -rotation period is 58.646 Earth days, Mercury revolves around the Sun -so rapidly that an observer on Mercury's equator would see a sunrise -only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury -day. Venus is more complicated, partly because its rotation is -slightly retrograde: its year is 1.92 of its days. Gas giants like -Jupiter are trickier still, as their polar and equatorial regions -rotate at different rates, so that the length of a day depends on -latitude. This effect is most pronounced on Neptune, where the day is -about 12 hours at the poles and 18 hours at the equator. +like Earth's. +On other planets, Sun-based time and calendars would work quite +differently. +For example, although Mercury's +<a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal +rotation period</a> is 58.646 Earth days, Mercury revolves around the +Sun so rapidly that an observer on Mercury's equator would see a +sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a +Mercury day. +Venus is more complicated, partly because its rotation is slightly +<a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>: +its year is 1.92 of its days. +Gas giants like Jupiter are trickier still, as their polar and +equatorial regions rotate at different rates, so that the length of a +day depends on latitude. +This effect is most pronounced on Neptune, where the day is about 12 +hours at the poles and 18 hours at the equator. </p> <p> -Although the tz database does not support time on other planets, it is -documented here in the hopes that support will be added eventually. +Although the <code><abbr>tz</abbr></code> database does not support +time on other planets, it is documented here in the hopes that support +will be added eventually. </p> <p> -Sources: +Sources for time on other planets: </p> + <ul> <li> -Michael Allison and Robert Schmunk, -"<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical -Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" -(2015-06-30). + Michael Allison and Robert Schmunk, + "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical + Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" + (2015-06-30). </li> <li> -Jia-Rui Chong, -"<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays -Fit for a Martian</a>", Los Angeles Times -(2004-01-14), pp A1, A20-A21. + Jia-Rui Chong, + "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays + Fit for a Martian</a>", <cite>Los Angeles Times</cite> + (2004-01-14), pp A1, A20-A21. </li> <li> -Tom Chmielewski, -"<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-…">Jet -Lag Is Worse on Mars</a>", The Atlantic (2015-02-26) + Tom Chmielewski, + "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-…">Jet + Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26) </li> <li> -Matt Williams, -"<a href="https://www.universetoday.com/37481/days-of-the-planets/">How -long is a day on the other planets of the solar system?</a>" -(2017-04-27). + Matt Williams, + "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How + long is a day on the other planets of the solar system?</a>" + (2017-04-27). </li> </ul> - </section> +</section> - <footer> - <hr> -This file is in the public domain, so clarified as of 2009-05-17 by -Arthur David Olson. - </footer> +<footer> + <hr> + This file is in the public domain, so clarified as of 2009-05-17 by + Arthur David Olson. +</footer> </body> </html> -- 2.7.4

1 0

Adding a "test" region?
by Paul G Jan. 26, 2018

Jan. 26, 2018

All this hubbub about the negative SAVE values in the tzdb source code makes it pretty obvious that downstream consumers are making some assumptions about undocumented features of the tzdb code. I'm wondering if it might be worth it to add a new region that contains fictional time zones that have properties that are perfectly valid but are edge cases that might be improperly handled downstream. It's probably a good idea to have this sort of thing in place *before* some region makes a change that actually violates one of these assumptions. A few example test cases I can think of (but I assume others will have *many* other ideas): - Zone with negative DST - Zone with large DST offset and/or large standard offset - Zone with 3, 4 or 5 offsets per year - Zone which goes from STD->DST with no change in offset. - Zone with new rules that don't take place until after 2038 - Zone with arbitrary DST offsets (10m15s, for example) - Zone that corresponds to TAI (e.g. offsets change to compensate for leap seconds) Downstream consumers could then use this in their test suites and be effectively "on notice" that they should be able to handle these sorts of cases, even though we have no examples of them existing in the wild (though there's actually some case to be made for interpreting TAI as a time zone, in which case that one *does* exist in the wild). There is some precedent for this sort of thing in the GREASE mechanism for testing TLS extensibility ( https://tools.ietf.org/html/draft-ietf-tls-grease-00 ), which provides some mechanism for testing whether TLS implementations will break if some arbitrary extensions are added. Best, Paul

12 31

Re: [tz] Irish Standard Time vs Irish Summer Time
by Paul Eggert Jan. 24, 2018

Jan. 24, 2018

On 01/23/2018 05:29 AM, Michael H Deckers wrote: > > The assertion > # ... but there's no > # convenient single notation for the date and time of this > transition > # so we must duplicate the Rule lines. > is clearly wrong when zic allows negative times of day with > the u suffix -- but it is still in australasia in 2018b. Good point. I reworded that in the attached patch, which also cleans up some other stale comments that I discovered while looking into this one. I installed this in the master branch, after 2018c was tagged. > Similarly, the (future) change in meaning for tm_isdst (from an > indicator of daylight saving time to one of non-standard time) > should be applied throughout the whole documentation of tzdb. > For instance, newctime.3.txt says > Tm_isdst is non-zero if summer time is in effect. > and this will probably become false (for the effect of > localtime()) after the change. It's probably better to consistently use the phrase "daylight saving time", as this is common usage (at least in the US) and it maps well to the tm_isdst variable of C. Logically DST can be observed in summer just as easily as in winter, and there's no logical requirement that it must advance the clock: after all, one can "save" daylight at the start of the day, and this is just as easy as "saving" it at the end.

2 1

Re: [tz] 2018c minor issue
by Paul Eggert Jan. 23, 2018

Jan. 23, 2018

On 01/23/2018 02:39 PM, Pascal Werz wrote: > I tried to build the 2018c release of tz code and data and found that > file zishrink.awk line 40: > gsub(/[\f\r\t\v ]+/, " ", line) > erroneously changed all 'v' characters to spaces as '\v' was not > recognized as an escape sequence (hence handled as 'v') on my OS. > Changing the line to > gsub(/[\f\r\t\013 ]+/, " ", line) > or removing '\v' from the regexp allowed the build to be possible again. Thanks for the bug report. I installed the attached, which should appear in the next release. POSIX requires awk to support \v but if mawk can stray from POSIX I suppose macOS awk can too.

2 1

[tz-announce] 2018c release of tz code and data available
by Paul Eggert Jan. 23, 2018

Jan. 23, 2018

The 2018c release of the tz code and data is available. It follows on the 2018a and 2018b releases, which were published but were not announced until now, due to problems discovered late in their release processes. 2018a had a build-failure typo, and 2018a and 2018b both had problems with ICU and Java, downstream packages which do not support a feature (negative DST offsets) used in 2018a and 2018b. The typo has been fixed, and data changes using negative DST offsets have been reverted pending development of a mechanism to export data to platforms lacking support for such data. The 2018a through 2018c releases reflect the following changes, which were either circulated on the tz mailing list or are relatively minor technical or administrative changes. This announcement has merged the set of changes made by the three releases, to make it easier to see the difference between 2017c and 2018c; please see the 2018c NEWS file for more details about intermediate versions. Release 2018c - 2018-01-22 23:00:44 -0800 Release 2018b - 2018-01-17 23:24:48 -0800 Release 2018a - 2018-01-12 22:29:21 -0800 Briefly: São Tomé and Príncipe switched from +00 to +01. Brazil's DST will now start on November's first Sunday. Use Debian-style installation locations, instead of 4.3BSD-style. New zic option -t. Changes to past and future time stamps São Tomé and Príncipe switched from +00 to +01 on 2018-01-01 at 01:00. (Thanks to Steffen Thorsen and Michael Deckers.) Changes to future time stamps Starting in 2018 southern Brazil will begin DST on November's first Sunday instead of October's third Sunday. (Thanks to Steffen Thorsen.) Changes to past time stamps Japanese DST transitions (1948-1951) were Sundays at 00:00, not Saturdays or Sundays at 02:00. (Thanks to Takayuki Nikai.) A discrepancy of 4 s in timestamps before 1931 in South Sudan has been corrected. The 'backzone' and 'zone.tab' files did not agree with the 'africa' and 'zone1970.tab' files. (Problem reported by Michael Deckers.) The abbreviation invented for Bolivia Summer Time (1931-2) is now BST instead of BOST, to be more consistent with the convention used for Latvian Summer Time (1918-9) and for British Summer Time. Changes to build procedure The default installation locations have been changed to mostly match Debian circa 2017, instead of being designed as an add-on to 4.3BSD circa 1986. This affects the Makefile macros TOPDIR, TZDIR, MANDIR, and LIBDIR. New Makefile macros TZDEFAULT, USRDIR, USRSHAREDIR, BINDIR, ZDUMPDIR, and ZICDIR let installers tailor locations more precisely. (This responds to suggestions from Brian Inglis and from Steve Summit.) The default installation procedure no longer creates the backward-compatibility link US/Pacific-New, which causes confusion during user setup (e.g., see Debian bug 815200). Use 'make BACKWARD="backward pacificnew"' to create the link anyway, for now. Eventually we plan to remove the link entirely. tzdata.zi now contains a version-number comment. (Suggested by Tom Lane.) The Makefile now quotes values like BACKWARD more carefully when passing them to the shell. (Problem reported by Zefram.) Builders no longer need to specify -DHAVE_SNPRINTF on platforms that have snprintf and use pre-C99 compilers. (Problem reported by Jon Skeet.) The build procedure now works around mawk 1.3.3's lack of support for character class expressions. (Problem reported by Ohyama.) Changes to code zic has a new option -t FILE that specifies the location of the file that determines local time when TZ is unset. The default for this location can be configured via the new TZDEFAULT makefile macro, which defaults to /etc/localtime. Diagnostics and commentary now distinguish UT from UTC more carefully; see theory.html for more information about UT vs UTC. zic has been ported to GCC 8's -Wstringop-truncation option. (Problem reported by Martin Sebor.) Changes to documentation and commentary The zic man page now documents the longstanding behavior that times and years can be out of the usual range, with negative times counting backwards from midnight and with year 0 preceding year 1. (Problem reported by Michael Deckers.) The theory.html file now mentions the POSIX limit of six chars per abbreviation, and lists alphabetic abbreviations used. The files tz-art.htm and tz-link.htm have been renamed to tz-art.html and tz-link.html, respectively, for consistency with other file names and to simplify web server configuration. Here are links to the release files: https://www.iana.org/time-zones/repository/releases/tzcode2018c.tar.gz https://www.iana.org/time-zones/repository/releases/tzdata2018c.tar.gz https://www.iana.org/time-zones/repository/releases/tzdb-2018c.tar.lz As usual, links to the latest release files are here: https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz Links are also available via plain HTTP, and via FTP fromftp://ftp.iana.org/tz/releases with the same basenames as above. Each release file has a GPG signature, which can be retrieved by appending ".asc" to the above URLs. Copies of these signatures are appended to this message. This release corresponds to commit ef4db72f1cbe1f7141f8aa012a624abb0a8d8059 dated 2018-01-22 23:00:44 -0800 and tagged '2018c' in the development GitHub repository at <https://github.com/eggert/tz>. Here are the SHA-512 checksums for the release files: 21988e876479e38661d41ea4c7b5218ba14b979739d7ba8d49a2d343bb9f37c654056ab21c046a6652715f012e4ca33c4aa109b1ec3ac5d0244dd3a7ea9ed6d2 tzcode2018c.tar.gz 0575c87c9ffcde7b7f62b0df928d8a0e8d93f832b5ef7227b3d8431686f851f1fd5372c7e768f2979830352433b31df7f3979a00c28ea3973a3a0987ebbfb835 tzdata2018c.tar.gz a1686902f81b4aceba494797e4eb8cdb8c1037ab7a27df9635b7a45ecfd90b8cf7b605a252ad1a8c67394a578f95aa9dff03071bc871120a98328de437c19b4f tzdb-2018c.tar.lz Here are the GPG checksums for the release files: -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAABAgAGBQJaZuAMAAoJEO2X6Q5iqn40NNsP/i+4WUmbLMMZnnR9FTvRUILo UAHYZRqC1KbdMppKeeyTL6thANvjpMjeYM8V54tzndzrMhBaPc8SpiwVgTevRtKZ wG9BSb8Hx/RDQeBWOjRwFx5/WQ+ha4aK0IFB8KB5gNFLOuyMn+9ZaxSzoEXmNWX0 Ddoizk516lpfGuhZCnPMVtDorebaAby0d3WMCxzEJ2w3gLOI4Aj8WVR2Cl5eTlA/ cpQ+bzLog5lnf+mzs/bJeMlfxPSskAlvIBYk905uPTAT/LDeod+T+BLBfcGEUb5J D64veBERj01NDwVLDRqHG0hdw8aPdMQyTLje6fI7sVh2cu4lGgA7QxHC+aTnmYjt 862/wX7dx2HtfuaIr+6eulAIR3+rix0CMndEXUGLtqnx4WySh5rT++V4crGcgzSH eRl2TOkBBxpYLIK6KtHd31V5WhJfTN2NQOZEFdrxy+Z2pPWZJrhcB9+0tYGoRVs0 ARQdm5I1w5JwiHO7sI0YHWCPZCVhDC2+/Izg668OUIdmFpqXanMAADkT5Qo9frlu 72cnL0MFMB4JCQwbqE1Xa1AOzz6bUG7zUfRDyAH6wZyHIxZgr5PmbbfUfoagOD0J /psdEJNsOUdg+wDou23UVRyt9/WIiZbXKWVTs14DMuqrHLUQAcFQtpYwb/U3z5mR 0I0syyatHweHMeftotgJ =gt0T -----END PGP SIGNATURE----- -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAABAgAGBQJaZuAMAAoJEO2X6Q5iqn40WkcP/3/QD5VP4YT5/zLPqTE4hLMB iuNFJelf1Iv4hFXYCMhfnU+0MlUfvSBBKLI5uu1vTl48ms33Ku+CL6fKI8uIlwMG v0yTCOxRozMofUcjsm2IfGRlJgAajMPWDOGsD3jocX0Vf14m9siycy4xdBk7vNrs QUKL3zl0XOBQY51ivnYvEK/f8LJZzn7NHizL0VwI0rxO/Oj6o0Q2wWkE61tBS27r znn9lFRWH1Nh+TJ7Yh1mNAd4UOYGL9Q3eoWkl5MzPM3ujjVSdsEXhNLGemPrfE50 RzuvsLgiKqahBpC9Zxy39pqiO+riRKHLlNjxNcCeHw6M5w8/FuBwwozSmdchWaZn gD50k7az4F5nojUgzYTNGV6nW2CGflLLgOeUWMmYUyF999KpOJzLXNFqMuBYqOOu ODI7CJweT0/Ff9jjoKMSqg1rMMDEox/E127ms8uf6qGyjWvp0AQlA5O06bStfpjH HDJog9BcDXb64oiJsIg0K4UqqF1n7jMqiufhz4WhrqPqaTP9qJuorSn5yar/UsGc V2l2C+BdJhmsvQ9vkLrYJNWXks+oQSFJ1dYYpv+zp/MsiBV1/pKaYEXgurRfQBCW eHO57ovm2Pf59dghTnm7SyoiZeqCK/89MWxpX4YbDgzP4ZyBhS2g1/bk7toPajUa idAZsIgmZDmw77UHpPJu =nVJg -----END PGP SIGNATURE----- -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAABAgAGBQJaZuAMAAoJEO2X6Q5iqn40rDcP/3cxJ0Ri2s3WNCnu1woSJ3KU zRc5lvC4YZRo8/4yqEjzcq8M2eiYs540WqE4kPm9dFSefCOWX6nw5imrU2cM9EfJ XPyrZho+r85uH5KMCgQ9J+AC1HBya4TPdaTPikEYVARBMmkwLtM34FxJz2VnFRhT ZQjDG+ncZNhOBXWu6Mughb6I38Cl19/Dn2iQJ9p0XTay6uytCMceZ+o52bugeWCL HlRkGqZgW9cxFygq3EBEGWN/IcsdFSZqpHLVslD5xeZP294tCW0LsgE/SZ0YP1c3 Dd0KWeC4iYbrETda57XA9+27siXwhT9hVQnXVeykmsmn+TftZHh89A+bVYlglhIi QXRH2pi9OX/8DgsTox3mswAM2OPFiUgdOthwVdnrdHjhWc74SFr7AARSmoIVZHve gKIQXCuTV+mcVKZ6ibHZHs86QQ/jevcLivlQdOO9MMHVNMu5ZjBR6E3QjY5TTwcW 2UT/JEMMluYA+b5xVrGOwWa2g2vU/K1nvOVX73vT3n552djrZ6NholegrVVbcVjO GySPxMwjm1MMlmvXnN0cr4eWzFamVdMMSBkn8X/aBjbHAIvnqiqoxK/9JQf0wEZV lgGETcAvxr540LUuY0R95SO3qy4MXrT5USMDUSnif8BsNoLe5MEExAmVJ3A7W1sP 3/xQouSXzl63eYI3Rvam =WEKZ -----END PGP SIGNATURE----- _______________________________________________ tz-announce mailing list tz-announce(a)iana.org https://mm.icann.org/mailman/listinfo/tz-announce

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Reason for removal of several TZ
by Thomas M Steenholdt Jan. 23, 2018

Jan. 23, 2018

Hi guys, Almost a year ago, commit 2999bb5bee719acbba8b9dd50fb9fb00c7788623 removed several TZ abbreviations and I'm trying to understand this. For my own use, what I'm after is WGT/WGST which are now "simply -03". WGT/WGST has been in tzdata for a long time, so the removal causes certain things to go belly up. The change was only implemented in Ubuntu recently, which is why this only turned into a problem for me now. WGT/WGST seems to be a widely accepted abbreviation for the West Greenland Time zone. So can someone explain to me the reason behind the removal and what would needed to get it re-implemented? Thanks. /Thomas

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Irish Standard Time vs Irish Summer Time
by Neil Fuller Jan. 22, 2018

Jan. 22, 2018

Just to weigh in from an Android OS perspective: the Europe/Dublin DST convention change is also problematic for the Android platform. Most of what I would want to say has been said. Like many, I missed the discussion in December about this change and I'm not sure I would have grasped the full impact if I had. I monitor the announcements list so it would have been most useful to me if this change had been flagged ahead of time there. I started looking at 2018a when I saw discussion internally to Google and I was surprised I hadn't seen an official announcement. It might help to understand the impact of this change and the indirect influence and interactions that the tzdata has in Android so I've included some information below on my ongoing assessment as an FYI. The NITZ standard[1] is probably the worst interaction I've seen so far. In the meantime Android will probably have to take the same approach that ICU has and patch the IANA data, or wait for 2018c to hopefully reverse the convention back to what it was before. As always, thanks to all of you for keeping the world turning! Neil. ------------- The latest Android code is downstream from both OpenJDK and ICU, with some of it's own time zone code on the side. There is a custom implementation of classes like java.util.TimeZone, but also code for time zone detection used in smartphone devices. Android will be exposed to any problems that the ICU/OpenJDK maintainers mentioned, plus any in other OS libraries used that I haven't looked at yet. Android uses tzdata in some places for transition calculations, but refers to ICU for things like names. There may be other interactions. If the tzdata had one interpretation, and ICU had another, Android's formatting code might return (for example) Irish Standard Time in Winter. I found a long-standing bug in the Android java.util.TimeZone code that forces the DST adjustment to always be >= 0 and another elsewhere that (obviously incorrectly) assumes DST always means +1 hour. The time zone detection code on Android can use the time zone offset data it has available on device and compare it against information sent by the carrier via NITZ[1] to see if it can identify the user's time zone. The NITZ documentation says: "When the LTZ is compensated for DST (summertime), the serving PLMN shall provide a DST parameter to indicate this. The adjustment for DST can be +1h or +2h." I'm looking into whether carriers / networks in Ireland have made the same interpretation as the new IANA data. I suspect their hands are tied by the NITZ standard which would probably prevent them passing a negative number. There's probably a feedback loop here between the NITZ standard, what carriers will send and what devices can accept that might take a while to stabilize on any new convention, if it happens at all. >From a support perspective, Android has three cohorts to deal with: 1) Devices running older versions of Android for which OEMs are still providing over-the-air (OTA) system image updates. These can potentially take code patches to correct issues. 2) Devices running older versions of Android that have the ability to receive time zone rules data updates but not code. 3) Latest code under development for future Android releases. If the Europe/Dublin change "sticks" in tzdata, the likely course for supporting existing devices would be to patch the Android tzdata and pretend the Europe/Dublin shift hasn't happened. Future devices could potentially support negative DST adjustments if all the problems are caught in advance. [1] https://en.wikipedia.org/wiki/NITZ , see reference 1 for the standard I quoted -- Google UK Limited Registered Office: 6 Pancras Square, London, N1C 4AG Registered in England Number: 3977902

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Changing times
by Arthur David Olson Jan. 22, 2018

Jan. 22, 2018

In days of old, there was only one consumer of the time zone data files: the time zone compiler that's part of the IANA package. Today there are multiple consumers. My sense: that multiplication predominates over any change in maintenance approach as a complicating factor. @dashdashado

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