How to Read the tz Database

From 8208572aae11a349eb269006f4ca72b880d48c42 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Mon, 29 Feb 2016 21:00:44 -0800 Subject: [PATCH 2/3] tz-how-to merge and fixups MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit * NEWS: Mention new web page. * Makefile (WEB_PAGES): Add tz-how-to.html. * tz-how-to.html: Make it pass ‘make check’. Update URLs. Put public-domain notice at bottom. Avoid long lines when possible. * tz-link.htm: Link to it. Also, fix some URLs (thanks to Sanjeev and to Brian Inglis). --- Makefile | 2 +- NEWS | 2 + tz-how-to.html | 770 ++++++++++++++++++++++++++++++--------------------------- tz-link.htm | 16 +- 4 files changed, 420 insertions(+), 370 deletions(-) diff --git a/Makefile b/Makefile index 5e2fb52..720c6ef 100644 --- a/Makefile +++ b/Makefile @@ -364,7 +364,7 @@ MANTXTS= newctime.3.txt newstrftime.3.txt newtzset.3.txt \ tzfile.5.txt tzselect.8.txt zic.8.txt zdump.8.txt \ date.1.txt COMMON= CONTRIBUTING LICENSE Makefile NEWS README Theory -WEB_PAGES= tz-art.htm tz-link.htm +WEB_PAGES= tz-art.htm tz-how-to.html tz-link.htm DOCS= $(MANS) date.1 $(MANTXTS) $(WEB_PAGES) PRIMARY_YDATA= africa antarctica asia australasia \ europe northamerica southamerica diff --git a/NEWS b/NEWS index e4dc2b5..c55d7af 100644 --- a/NEWS +++ b/NEWS @@ -33,6 +33,8 @@ Unreleased, experimental changes tzselect again limits its menu comments so that menus fit on a 24x80 alphanumeric display. + A new web page tz-how-to.html. (Thanks to Bill Seymour.) + In the Theory file, the description of possible time zone abbreviations in tzdata has been cleaned up, as the old description was unclear and inconsistent. (Thanks to Alain Mouette for reporting the problem.) diff --git a/tz-how-to.html b/tz-how-to.html index 544ef7b..3fa7602 100644 --- a/tz-how-to.html +++ b/tz-how-to.html @@ -1,539 +1,582 @@ + How to Read the tz Database -

How to Read the tz Database Source Files

How to Read the tz +Database Source Files

by Bill Seymour

This paper is in the public domain.

- -This paper uses the America/Chicago and Pacific/Honolulu zones as examples of how to infer -times of day from the tz database -source files. It might be helpful, but not absolutely necessary, -for the reader to have already downloaded the database from -ftp://elsie.nci.nih.gov/pub/ -and become familiar with the basic layout of the data files. The format is explained -in the “man page” for the zic compiler, zic.8.txt, in the code -subdirectory. +

This page uses the America/Chicago and +Pacific/Honolulu zones as examples of how to infer +times of day from the tz database +source files. It might be helpful, but not absolutely necessary, +for the reader to have already downloaded the +latest +release of the database and become familiar with the basic layout +of the data files. The format is explained in the “man +page” for the zic compiler, zic.8.txt, in +the code subdirectory.

We’ll begin by talking about the rules for changing between standard and daylight saving time since we’ll need that information when we talk -about the zones. +about the zones.

+ +

First, let’s consider the special daylight saving time rules +for Chicago (from the northamerica file in +the data subdirectory):

First, let’s consider the special daylight saving time rules for Chicago -(from the northamerica file in the data subdirectory): -

- +
- + - - + - + - - - - + + + + - + - + - - + + - + - - + + - + - + - +

From the Source File From the Source File

+ +

- -# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER -Rule Chicago 1920 only - Jun 13 2:00 1:00 D -Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S -Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D -Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D -Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S -Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S +#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER +Rule Chicago 1920 only - Jun 13 2:00 1:00 D +Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S +Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D +Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D +Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S +Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
-

Reformatted a Bit Reformatted a Bit

From To On On At Action
1920 only June 13^th 02:00 local
1920 only June 13^th 02:00 local go to daylight saving time
1920 1921 last Sunday last Sunday in October return to standard time
1921 only
1921 only in March go to daylight saving time go to daylight saving time
1922
1922 1966 in April
1954 in September return to standard time return to standard time
1955 1966 in October

-

We’ll basically just ignore the TYPE column. +

We’ll basically just ignore the TYPE column. In the 2007j release, the most recent as of this writing, the -TYPE column never contains anything but a hyphen, -a kind of null value. (From the description in zic.8.txt, +TYPE column never contains anything but a hyphen, +a kind of null value. (From the description in zic.8.txt, this appears to be a mechanism for removing years from a set -in some localizable way. It’s used in the file, pacificnew, +in some localizable way. It’s used in the file, pacificnew, to determine whether a given year will have a US presidential election; but everything related to that use is commented out.) -

The SAVE column contains the wall clock offset from local standard time. +

The SAVE column contains the wall clock offset from +local standard time. This is usually either zero for standard time or one hour for daylight saving time; but there’s no reason, in principle, why it can’t take on other values. -

The LETTER (sometimes called LETTER/S) column can contain a variable +

The LETTER (sometimes called LETTER/S) +column can contain a variable part of the usual abbreviation of the time zone’s name, or it can just -be a hyphen if there’s no variable part. For example, the abbreviation +be a hyphen if there’s no variable part. For example, the abbreviation used in the central time zone will be either “CST” or -“CDT”. The variable part is ‘S’ or ‘D’; -and, sure enough, that’s just what we find in the LETTER column -in the Chicago rules. More about this when we talk about +“CDT”. The variable part is ‘S’ or ‘D’; +and, sure enough, that’s just what we find in +the LETTER column +in the Chicago rules. More about this when we talk about “Zone” lines.

One important thing to notice is that “Rule” lines want at once to be both transitions and steady states:

On the one hand, they represent transitions between standard and daylight saving time; - and any number of Rule lines can be in effect during a given period (which will always - be a non-empty set of contiguous calendar years). -
-
On the other hand, the SAVE and LETTER columns contain - state that exists between transitions. More about this when we talk about - the US rules. +
On the one hand, they represent transitions between standard and +daylight saving time; and any number of Rule lines can be in effect +during a given period (which will always be a non-empty set of +contiguous calendar years).
On the other hand, the SAVE and LETTER +columns contain state that exists between transitions. More about this +when we talk about the US rules.

In the example above, the transition to daylight saving time happened on the -13^th of June in 1920, and on the last Sunday in March -in 1921; but the return to standard time happened on the last Sunday in October -in both of those years. Similarly, the rule for changing to daylight saving time -was the same from 1922 to 1966; but the rule for returning to standard time -changed in 1955. Got it? +

In the example above, the transition to daylight saving time +happened on the 13^th of June in 1920, and on +the last Sunday in March in 1921; but the return to standard time +happened on the last Sunday in October in both of those +years. Similarly, the rule for changing to daylight saving time was +the same from 1922 to 1966; but the rule for returning to standard +time changed in 1955. Got it?

OK, now for the somewhat more interesting “US” rules: -

- +
OK, now for the somewhat more interesting “US” rules:
+ +
- + - - + - + - - - - + + + + - + - + - - - + + + - - - - + + + + - - - - + + + + - - + + - + - + - + - - - + + + - - - + + + - + - + - + - - - - + + + + - - + +

From the Source File From the Source File

+ +

- -# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S -Rule US 1918 1919 - Mar lastSun 2:00 1:00 D -Rule US 1918 1919 - Oct lastSun 2:00 0 S -Rule US 1942 only - Feb 9 2:00 1:00 W # War -Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace -Rule US 1945 only - Sep 30 2:00 0 S -Rule US 1967 2006 - Oct lastSun 2:00 0 S -Rule US 1967 1973 - Apr lastSun 2:00 1:00 D -Rule US 1974 only - Jan 6 2:00 1:00 D -Rule US 1975 only - Feb 23 2:00 1:00 D -Rule US 1976 1986 - Apr lastSun 2:00 1:00 D -Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D -Rule US 2007 max - Mar Sun>=8 2:00 1:00 D -Rule US 2007 max - Nov Sun>=1 2:00 0 S +#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S +Rule US 1918 1919 - Mar lastSun 2:00 1:00 D +Rule US 1918 1919 - Oct lastSun 2:00 0 S +Rule US 1942 only - Feb 9 2:00 1:00 W # War +Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace +Rule US 1945 only - Sep 30 2:00 0 S +Rule US 1967 2006 - Oct lastSun 2:00 0 S +Rule US 1967 1973 - Apr lastSun 2:00 1:00 D +Rule US 1974 only - Jan 6 2:00 1:00 D +Rule US 1975 only - Feb 23 2:00 1:00 D +Rule US 1976 1986 - Apr lastSun 2:00 1:00 D +Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D +Rule US 2007 max - Mar Sun>=8 2:00 1:00 D +Rule US 2007 max - Nov Sun>=1 2:00 0 S
-

Reformatted a Bit Reformatted a Bit

From To On On At Action
1918 1919 last Sunday
1918 1919 last Sunday in March 02:00 local 02:00 local go to daylight saving time
in October return to standard time
1942 only February 9^th
1942 only February 9^th go to “war time”
1945 only August 14^th 23:00 UT
1945 only August 14^th 23:00 UT - rename “war time” to “peace
time;” clocks don’t change + rename “war time” to “peace
time;” + clocks don’t change
September 30^th 02:00 local return to standard time
September 30^th 02:00 local return to standard time
1967
1967 2006 last Sunday last Sunday in October
1973 in April go to daylight saving time go to daylight saving time
1974 only January 6^th
1974 only January 6^th
1975 only February 23^rd
1975 only February 23^rd
1976 1986 last Sunday in April in April
1987 2006 first Sunday
2007 present second Sunday in March
2007 present second Sunday in March
first Sunday in November
first Sunday in November return to standard time

-

There are two interesting things to note here. - -

First, the time that something happens (in the AT column) is not necessarily -the local wall clock time. The time can be suffixed with ‘s’ (for -“standard”) to mean local standard time (different from wall clock -time when observing daylight saving time); or it can be suffixed with ‘g’, -‘u’, or ‘z’, all three of which mean the standard time at the -prime meridan. ‘g’ -stands for “GMT”; -‘u’ -stands for “UT” or -“UTC” -(whichever was official at the time); -‘z’ stands for the -nautical time zone Z -(a.k.a. “Zulu” which, in turn, stands for ‘Z’). - -The time can also be suffixed with -‘w’ meaning “wall clock time;” but it usually isn’t -because that’s the default. - -

Second, the day in the ON column, in addition to “lastSun” or a particular -day of the month, can have the form, “Sun>=x” -or “Sun<=x,” -where x is a day of the month. For example, “Sun>=8” -means “the first Sunday on or after the eighth of the month,” in other words, the second Sunday -of the month. Furthermore, although there are no examples above, the weekday needn’t be -“Sun” in either form, but can be the usual three-character -English abbreviation for any day of the week. - -

And the US rules give us more examples of a couple of things already mentioned: +

There are two interesting things to note here.

+ +

First, the time that something happens (in the AT +column) is not necessarily the local wall clock time. The time can be +suffixed with ‘s’ (for “standard”) to mean +local standard time (different from wall clock time when observing +daylight saving time); or it can be suffixed with ‘g’, +‘u’, or ‘z’, all three of which mean the +standard time at the +prime meridan. +‘g’ stands for “GMT”; +‘u’ stands for “UT” or “UTC” +(whichever was official at the time); ‘z’ stands for the +nautical time zone +Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). +The time can also be suffixed with ‘w’ meaning “wall +clock time;” but it usually isn’t because that’s the +default.

+ +

Second, the day in the ON column, in addition to +“lastSun” or a particular day of the month, +can have the form, “Sun>=x” or +“Sun<=x,” where x is a day +of the month. For example, “Sun>=8” means +“the first Sunday on or after the eighth of the month,” in +other words, the second Sunday of the month. Furthermore, although +there are no examples above, the weekday needn’t be +“Sun” in either form, but can be the usual +three-character English abbreviation for any day of the week.

+ +

And the US rules give us more examples of a couple of things +already mentioned:

The rules for changing to and from daylight saving time are actually different sets - of rules; and the two sets can change independently. Consider, for example, that the rule - for the return to standard time stayed the same from 1967 to 2006; but the rule for - the transition to daylight saving time changed several times in the same period. - There can also be periods, 1946 to 1966 for example, when no rule from this group - is in effect, and so either no transition happened in those years, or some other - rule is in effect (perhaps a state or other more local rule). -
-
The SAVE and LETTER columns contain steady state, not transitions. - Consider, for example, the transition from “war time” to “peace time” - that happened on August 14, 1945. The “1:00” in the SAVE column - is not an instruction to advance the clock an hour. It means that clocks should - be one hour ahead of standard time, which they already are because of the previous rule, - so there should be no change. +
The rules for changing to and from daylight saving time are +actually different sets of rules; and the two sets can change +independently. Consider, for example, that the rule for the return to +standard time stayed the same from 1967 to 2006; but the rule for the +transition to daylight saving time changed several times in the same +period. There can also be periods, 1946 to 1966 for example, when no +rule from this group is in effect, and so either no transition +happened in those years, or some other rule is in effect (perhaps a +state or other more local rule).
The SAVE and LETTER columns +contain steady state, not transitions. Consider, for example, +the transition from “war time” to “peace time” +that happened on August 14, 1945. The “1:00” in +the SAVE column is not an instruction to advance +the clock an hour. It means that clocks should be one hour +ahead of standard time, which they already are because of the previous +rule, so there should be no change.

OK, now let’s look at a Zone record: -

- +
OK, now let’s look at a Zone record:
+ +
- + - - + - - - - + + + + - + - - + + - + - + - + - + - - + + - + - + - + - + - + - +

From the Source File From the Source File

+ +

- -# Zone NAME GMTOFF RULES FORMAT [UNTIL] -Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 - -6:00 US C%sT 1920 - -6:00 Chicago C%sT 1936 Mar 1 2:00 - -5:00 - EST 1936 Nov 15 2:00 - -6:00 Chicago C%sT 1942 - -6:00 US C%sT 1946 - -6:00 Chicago C%sT 1967 - -6:00 US C%sT +#Zone NAME GMTOFF RULES FORMAT [UNTIL] +Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 + -6:00 US C%sT 1920 + -6:00 Chicago C%sT 1936 Mar 1 2:00 + -5:00 - EST 1936 Nov 15 2:00 + -6:00 Chicago C%sT 1942 + -6:00 US C%sT 1946 + -6:00 Chicago C%sT 1967 + -6:00 US C%sT
-

Columns Renamed Columns Renamed

Standard Offset
from Prime Meridian Daylight
Saving Time Abbreviation(s) Ending at Local Time Standard Offset
+ from Prime + Meridian Daylight
Saving Time Abbreviation(s) Ending at Local Time

Date Time
−5:50:36 not observed LMT 1883-11-18 12:09:24
−6:00:00
−6:00:00 US rules CST or CDT CST or CDT 1920-01-01 00:00:00
Chicago rules 1936-03-01 02:00:00 02:00:00
−5:00:00 not observed EST 1936-11-15
−6:00:00
−6:00:00 Chicago rules CST or CDT 1942-01-01 00:00:00 00:00:00
US rules CST, CWT or CPT 1946-01-01
Chicago rules CST or CDT CST or CDT 1967-01-01
US rules — —

-

There are a couple of interesting differences between Zones and Rules. - -

First, and somewhat trivially, whereas Rules are considered to contain one or more -records, a Zone is considered to be a single record with zero or more continuation -lines. Thus, the keyword, “Zone,” and the zone name -are not repeated. The last line is the one without anything in the [UNTIL] column. - -

Second, and more fundamentally, each line of a Zone represents -a steady state, not a transition between states. The state exists from the date and time -in the previous line’s [UNTIL] column up to the -date and time in the current line’s [UNTIL] column. In other words, -the date and time in the [UNTIL] column is the instant that separates -this state from the next. Where that would be ambiguous because we’re -setting our clocks back, the [UNTIL] column specifies the first occurrence of -the instant. The state specified by the last line, the one without -anything in the [UNTIL] column, continues to the present. - -

The first line typically specifies the mean solar time observed before the introduction -of standard time. Since there’s no line before that, it has no beginning. 8-) -For some places near the -International Date Line, -the first two lines will show solar times differing by 24 hours; this corresponds to -a movement of the Date Line. -For example: +

There are a couple of interesting differences between Zones and Rules.

+ +

First, and somewhat trivially, whereas Rules are considered to +contain one or more records, a Zone is considered to be a single +record with zero or more continuation lines. Thus, the keyword, +“Zone,” and the zone name are not +repeated. The last line is the one without anything in +the [UNTIL] column.

+ +

Second, and more fundamentally, each line of a Zone represents a +steady state, not a transition between states. The state exists from +the date and time in the previous line’s [UNTIL] +column up to the date and time in the current +line’s [UNTIL] column. In other words, the date and +time in the [UNTIL] column is the instant that separates +this state from the next. Where that would be ambiguous because +we’re setting our clocks back, the [UNTIL] column +specifies the first occurrence of the instant. The state specified by +the last line, the one without anything in the [UNTIL] +column, continues to the present.

+ +

The first line typically specifies the mean solar time observed +before the introduction of standard time. Since there’s no line before +that, it has no beginning. 8-) For some places near the International +Date Line, the first two lines will show solar times +differing by 24 hours; this corresponds to a movement of the Date +Line. For example:

-# Zone        NAME                GMTOFF        RULES        FORMAT        [UNTIL]
-Zone America/Juneau         15:02:19 -        LMT        1867 Oct 18
-                         -8:57:41 -        LMT        ...
+#Zone NAME          GMTOFF   RULES FORMAT [UNTIL]
+Zone America/Juneau 15:02:19 -     LMT    1867 Oct 18
+                    -8:57:41 -     LMT    ...

-When Alaska was purchased from Russia in 1867, the Date Line moved from -the Alaska/Canada border to -the Bering Strait; and the time in Alaska was then 24 hours earlier than it had been. -<aside>(6 October in the Julian calendar, -which Russia was still using then for religious reasons, was followed by a second instance -of the same day with a different name, 18 October -in the Gregorian calendar. Isn’t civil time -wonderful? 8-))</aside> - -

The abbreviation, “LMT,” stands for “local mean time,” -which is an invention of the tz database -and was probably never actually used during the period. Furthermore, the value -is almost certainly wrong except in the archetypal place after which the zone -is named. (The tz database usually doesn’t provide a separate Zone record -for places where nothing significant happened after 1970.) - -

The RULES column tells us whether daylight saving time is being observed: +

When Alaska was purchased from Russia in 1867, the Date Line moved +from the Alaska/Canada border to the Bering Strait; and the time in +Alaska was then 24 hours earlier than it had +been. <aside>(6 October in the Julian calendar, +which Russia was still using then for religious reasons, was followed +by a second instance of the same day with a different name, 18 +October in the Gregorian calendar. Isn’t civil time +wonderful? 8-))</aside>

+ +

The abbreviation, “LMT” stands for “local mean +time”, which is an invention of +the tz +database and was probably never actually used during the +period. Furthermore, the value is almost certainly wrong except in the +archetypal place after which the zone is named. (The tz database +usually doesn’t provide a separate Zone record for places where +nothing significant happened after 1970.)

+ +

The RULES column tells us whether daylight saving time is being observed:

A hyphen, a kind of null value, means that we have not set our clocks ahead of standard time. -
An amount of time (usually but not necessarily “1:00” - meaning one hour) means that we have set our clocks ahead by that amount. -
Some alphabetic string means that we might have set our clocks ahead; - and we need to check the rule the name of which is the given alphabetic string. +
A hyphen, a kind of null value, means that we have not set our +clocks ahead of standard time.
An amount of time (usually but not necessarily “1:00” +meaning one hour) means that we have set our clocks ahead by that +amount.
Some alphabetic string means that we might have set our +clocks ahead; and we need to check the rule the name of which is the +given alphabetic string.

-An example of a specific amount of time is: +

An example of a specific amount of time is:

-# Zone        NAME                GMTOFF        RULES        FORMAT        [UNTIL]
-Zone Pacific/Honolulu        ...                        1933 Apr 30 2:00
-                        -10:30        1:00        HDT        1933 May 21 2:00
-                        ...
+#Zone NAME            GMTOFF RULES FORMAT [UNTIL]
+Zone Pacific/Honolulu ...                 1933 Apr 30 2:00
+                      -10:30 1:00  HDT    1933 May 21 2:00
+                      ...

-Hawaii tried daylight saving time for three weeks in 1933 and decided they didn’t -like it. 8-) Note that the GMTOFF column always contains -the standard time offset, so the wall clock time during this period was -GMT − 10:30 + 1:00 = GMT − 9:30. -

The FORMAT column specifies the usual abbreviation of the time zone name. -It can have one of four forms: +

Hawaii tried daylight saving time for three weeks in 1933 and +decided they didn’t like it. 8-) Note that +the GMTOFF column always contains the standard time +offset, so the wall clock time during this period was GMT − +10:30 + 1:00 = GMT − 9:30.

+ +

The FORMAT column specifies the usual abbreviation of +the time zone name. It can have one of four forms:

the string, “zzz,” which is a kind of null value (don’t ask) -
a single alphabetic string other than “zzz,” - in which case that’s the abbreviation -
a pair of strings separated by a slash (‘/’), - in which case the first string is the abbreviation for the standard time name - and the second string is the abbreviation for the daylight saving time name -
a string containing “%s,” in which case the - “%s” will be replaced by the text in the - appropriate Rule’s LETTER column + +
the string, “zzz,” which is a kind of +null value (don’t ask)
a single alphabetic string other than “zzz,” +in which case that’s the abbreviation
a pair of strings separated by a slash +(‘/’), in which case the first string is the +abbreviation for the standard time name and the second string is the +abbreviation for the daylight saving time name
a string containing “%s,” in which case +the “%s” will be replaced by the text in the +appropriate Rule’s LETTER column

-The last two make sense only if there’s a named rule in effect. -

An example of a slash is: +

The last two make sense only if there’s a named rule in effect.

+ +

An example of a slash is:

-# Zone        NAME                GMTOFF        RULES        FORMAT        [UNTIL]
-Zone        Europe/London        ...                        1996
-                         0:00        EU        GMT/BST
+#Zone NAME          GMTOFF RULES FORMAT  [UNTIL]
+Zone  Europe/London ...                  1996
+                    0:00   EU    GMT/BST

-The current time in the UK is called either Greenwich mean time or British summer time. -

One wrinkle, not fully explained in zic.8.txt, is what happens when -switching to a named rule. To what values should the SAVE and LETTER -data be initialized? +

The current time in the UK is called either Greenwich mean time or +British summer time.

+ +

One wrinkle, not fully explained in zic.8.txt, is what +happens when switching to a named rule. To what values should +the SAVE and LETTER data be initialized?

If at least one transition has happened, use the SAVE and LETTER - data from the most recent. -
-
If switching to a named rule before any transition has happened, assume standard - time (SAVE zero), and use the LETTER data from the earliest - transition with a SAVE of zero. +
If at least one transition has happened, use +the SAVE and LETTER data from the most +recent.
If switching to a named rule before any transition has happened, +assume standard time (SAVE zero), and use +the LETTER data from the earliest transition with +a SAVE of zero. +

And two last things about the FORMAT column: +

And two last things about the FORMAT column:

The tz database gives - abbreviations for time zone names in popular usage, which is not - necessarily “correct” by law. For example, the last line in - Zone Pacific/Honolulu (shown below) gives - “HST” for “Hawaii standard time” even though the - legal - name for that time zone is “Hawaii-Aleutian standard time.” - This author has read that there are also some places in Australia where - popular time zone names differ from the legal ones. -
-
No attempt is made to - localize - the abbreviations. They are intended to be the values returned through the - "%Z" format specifier to - C’s - strftime - function in the - “C” locale. + +
The tz +database gives abbreviations for time zone names in popular +usage, which is not necessarily “correct” by law. For +example, the last line in +Zone Pacific/Honolulu (shown below) gives +“HST” for “Hawaii standard time” even though the +legal +name for that time zone is “Hawaii-Aleutian standard time.” +This author has read that there are also some places in Australia where +popular time zone names differ from the legal ones. + +
No attempt is made to localize +the abbreviations. They are intended to be the values returned through the +"%Z" format specifier to +C’s +strftime +function in the +“C” locale.

As a final example, here’s the complete history for Hawaii: +

As a final example, here’s the complete history for Hawaii:

- +
- + - - + -

Relevant Excerpts from the US Rules Relevant Excerpts from the US Rules

+ +

- -# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S -Rule US 1918 1919 - Oct lastSun 2:00 0 S -Rule US 1942 only - Feb 9 2:00 1:00 W # War -Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace -Rule US 1945 only - Sep 30 2:00 0 S +#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S +Rule US 1918 1919 - Oct lastSun 2:00 0 S +Rule US 1942 only - Feb 9 2:00 1:00 W # War +Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace +Rule US 1945 only - Sep 30 2:00 0 S
-

The Zone Record The Zone Record

- + - - - - + + + + @@ -541,7 +584,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 - + @@ -549,80 +592,85 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 - + - + - + - + - - + + - + - + - + - - + + - + - + - - -
+

- -# Zone NAME GMTOFF RULES FORMAT [UNTIL] -Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 - -10:30 - HST 1933 Apr 30 2:00 - -10:30 1:00 HDT 1933 May 21 2:00 - -10:30 US H%sT 1947 Jun 8 2:00 - -10:00 - HST +#Zone NAME GMTOFF RULES FORMAT [UNTIL] +Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 + -10:30 - HST 1933 Apr 30 2:00 + -10:30 1:00 HDT 1933 May 21 2:00 + -10:30 US H%sT 1947 Jun 8 2:00 + -10:00 - HST

What We Infer What We Infer

Wall-Clock
Offset from
Prime Meridian Adjust
Clocks Time Zone Ending at Local Time Wall-Clock
Offset from
Prime Meridian Adjust
Clocks Time Zone Ending at Local Time

Abbrv. Date Time
−10:31:26 — LMT 1900-01-01 12:00
−10:30 +0:01:26 HST Hawaii standard time 1933-04-30 02:00 02:00
−9:30 +1:00 HDT Hawaii daylight time 1933-05-21
−10:30¹ −1:00¹ HST¹ Hawaii standard time 1942-02-09
−9:30
−9:30 +1:00 HWT Hawaii war time 1945-08-14 13:30²
0 HPT Hawaii peace time 1945-09-30 02:00 02:00
−10:30 −1:00 HST Hawaii standard time HST Hawaii standard time 1947-06-08
−10:00³ +0:30³ — —

+ ¹Switching to US rules…most recent transition (in 1919) was to standard time

- ²23:00 UT + + ²23:00 UT + (−9:30) = 13:30 local

- ³Since 1947–06–08T12:30Z, + + ³Since 1947–06–08T12:30Z, the civil time in Hawaii has been - UT/UTC + UT/UTC − 10:00 year-round.

-
There will be a short quiz later. 8-) +
There will be a short quiz later. 8-)
-
+
+
+This web page is in the public domain, so clarified as of +2015-10-20 by Bill Seymour. +
All suggestions and corrections will be welcome; all flames will be amusing. Mail to was at pobox dot com. +
diff --git a/tz-link.htm b/tz-link.htm index bd92f64..f0bbf70 100644 --- a/tz-link.htm +++ b/tz-link.htm @@ -10,7 +10,7 @@ content="Sources for time zone and daylight saving time data"> - + The article tz database is an encyclopedic summary. -
How to Read the +
How to Read the tz Database Source Files explains the tz database format.

href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway lets you see the TZ values directly.

Current +href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current Time in 1000 Places uses descriptions of the values.

Time Zone Converter @@ -209,7 +209,7 @@ uses a pulldown menu.

Complete timezone information for all countries displays tables of DST rules.
The World Clock – -Time Zones lets you sort zone names and convert times.
+Worldwide lets you sort zone names and convert times.
Time Difference calculates the current time difference between locations.

Weather Now lists the weather too.
@@ -642,7 +642,7 @@ is Singapore in the "Wrong" Time Zone? details the history of legal time in Singapore and Malaysia.
United Kingdom

History of +href="https://www.polyomino.org.uk/british-time/">History of legal time in Britain discusses in detail the country with perhaps the best-documented history of clock adjustments. The National Physical Laboratory also maintains an Recent +href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent Time Zone Proceedings lists changes to time zone boundaries.

Precision timekeeping
@@ -699,7 +699,7 @@ code for converting among time scales like TAI, TDB, TDT and UTC. -
Basics of +
Basics of Space Flight – Reference Systems – Time Conventions briefly explains interplanetary space flight timekeeping.

ISO 8601 profile for use in new Internet protocols.

-Date & Time +Date & Time Formats on the Web surveys web- and Internet-oriented date and time formats.

-- 2.5.0