- tz - lists.iana.org

European time zones [forwarded message from cgcha!wtho]
by Arthur David Olson Feb. 4, 1987

Feb. 4, 1987

I've attached a note I received from Tom Hofmann, slightly edited at Tom's request. --ado ... Date: Wed, 28 Jan 87 16:56:27 -0100 From: seismo!mcvax!cgcha!wtho (Tom Hofmann) Message-Id: <8701281556.AA22174(a)cgcha.uucp> ... Subject: Time zone handling ... ... Let me say something about European time rules. First of all, I'm surprised to find the zone MET for Central Europe in file 'europe'. The correct expression is CET (Central European Time), although it's not the first time I found the expression MET. Maybe the best is a link between MET and CET. Secondly, the European time rules are much more complex than described in 'europe'. It is standardized since 1981, when most European countries started DST. Before that year, only a few countries (UK, France, Italy) had DST, each according to national rules. In 1981, however, DST started on 'Apr firstSun', and not on 'Mar lastSun' as in the following years. So we should set 1981 for FROM, and ignore the past. But also since 1981 there are some more national exceptions than listed in 'europe': Switzerland, for example, joined DST one year later, Denmark ended DST on 'Oct 1' instead of 'Sep lastSun' in 1981---I don't know how they handle things now. Finally, DST is always from 'Apr 1' to 'Oct 1' in the Soviet Union (as far as I know). 'europe' should be corrected in that sense, but perhaps you better make a query to Usenet first---maybe you'll find somone who knows more about national exceptions to European time rules. Tom Hofmann, Scientific Computer Center, CIBA-GEIGY AG, 4002 Basle, Switzerland UUCP: ...!mcvax!cernvax!cgcha!wtho

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Re: mktime
by seismo!cbpavo.MIS.OH.ATT.COM!mark Feb. 1, 1987

Feb. 1, 1987

I was VERY VERY happy to see mktime, as it turned out to be just what I needed for an application I'm supporting. (Actually, I had to alter it somewhat - the major change was that mktime speaks local time, and I needed a function that spoke UTC. Easy when I had the source, but it's worth mentioning that the UTC flavor is also useful.) What did I need it for? Well, I'm supporting a set of routines that provide X.410 style transactions over a network. These transactions have time stamps in them, and the time stamps are in ASCII, in a format that conforms (almost) to the appropriate ISO standards. These routines are mostly a cut up ctime.c, but I also had to supply functions to take strings received over the network (which are always in UTC) and do things like convert them to local time (for logs and for machines that keep time in local time), add N hours, etc. I'm enclosing the manual pages for my routines (the code itself is, I'm sorry to say, considered proprietary to AT&T, but the interface is open.) I consider this just an expedient solution, and not necessarily the end-all wonderful solution. Note that it does not provide a routine like mktime to the users, since I want to hide the time_t time from the users. (We have to worry about birth dates of parents of people alive today, so the 32 bit time_t isn't adequate. I don't really support that yet, but I don't provide user access to time_t so they won't write code now they'll be sorry for later.) Mark .TH ISODATE 3I .SH NAME isonow \- create ISO format string describing current date/time isodcmp \- compare two ISO format dates iso2local \- convert ISO date to local time iso2gmt \- convert ISO date to UTC time isoadd \- add a fixed offset to an ISO date .SH SYNOPSIS .nf .B "char *" .B "isonow();" .sp .B "int" .B "isodcmp(d1, d2);" .B "char *d1, *d2;" .sp .B "char *" .B "iso2local(date)" .B "char *date;" .sp .B "char *" .B "iso2gmt(date)" .B "char *date;" .sp .B "char *" .B "isoadd(date, days, hours, minutes, seconds)" .B "char *date;" .B "int days, hours, minutes, seconds" .fi .SH DESCRIPTION .P These routines manipulate ISO format date/time strings, as defined in the MDTP specification. Other routines may be added, as needed, later. .P .I isonow returns a character string, in ISO format, describing the current date and time. Only one of the ISO choices is used, namely: .nf yyyymmddhhmmsszzzzzE for example 19850410142954-0500E or 19850410142954+0930E or 19850410192954ZE .fi The first date is in the USA Eastern time zone, the second in the Australian Central time zone, and since isonow always returns GMT, the third sample is more typical. The trailing E means .I exact , as opposed to B for .I before or P for .I post (after.) .P .I isodcmp compares two ISO format times, telling which is earlier. It returns \-1 if the first is earlier, 0 if they are the same, and +1 if the second is earlier. .P .I iso2local takes an ISO format date (in either Z time zone, local time, or any other time zone) and converts it to the local time zone. It returns a string in ISO format indicating the local time zone. .P .I iso2gmt takes an ISO format date (in any time zone) and converts it to the Z (GMT) time zone. It returns a string in ISO format indicating the Z time zone. .P .I isoadd takes an ISO format date (in any time zone) and adds the given number of days, hours, minutes, and seconds to it. The numbers may be positive or negative. It returns a string in ISO format indicating the Z time zone. .SH BUGS In the interests of speed, .I isodcmp does not work correctly if any other variant of ISO format is used (e.g. seconds are missing, or a Z time zone is used, or a precision is included), or if the two time zones are not the same.

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a timezone list addition
by seismo!ut-sally!zotz.cs.utexas.edu!jsq Jan. 31, 1987

Jan. 31, 1987

>From: ames!scubed!ncr-sd!greg(a)sally.utexas.edu To: milano!im4u!ut-sally!std-unix(a)sally.utexas.edu Date: Thu, 15 Jan 87 16:16:56 pst Message-Id: <8701160016.AA04029@scubed> Subject: Re: time zone mailing list Newsgroups: mod.std.unix In-Reply-To: <6835(a)ut-sally.UUCP> Organization: NCR Corporation, Rancho Bernardo In article <6835(a)ut-sally.UUCP> you write: >Any interested mod.std.unix readers can mail me a request to be added. Sigh. I'm not sure I should do this after my last attempt at commentary met with such disaster, but could you put me on the list? Tks. Oh, I'm Greg.Noel(a)SanDiego.NCR.COM or greg@ncr-sd, whichever your software will accept. -- -- Greg Noel, NCR Rancho Bernardo Greg(a)ncr-sd.UUCP or Greg(a)nosc.ARPA

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mktime
by Arthur David Olson Jan. 30, 1987

Jan. 30, 1987

Now that new requests for addition to the tz mailing list have tapered off (and now that the snows have stopped here in the Washington, D.C. area) I'd like to note a problem in the October, 1986 version of the Rationale document that accompanies the draft proposed C standard, and then ask some questions. The part of the Rationale document that I'm troubled by is section 4.12.2.3 on the mktime function: > mktime is an invention of the Committee to complete the set of time > functions. With this function it becomes possible to perform portable > calculations involving clock times and broken-down times. > > The rules on the ranges of the fields within the *timeptr > record are crafted to permit useful arithmetic to be done. For > instance, here is a paradigm for continuing some loop for an hour: > > #include <time.h> > struct tm when; > time_t now; > time_t deadline; > > /* ... */ > time(now); > when = *localtime(now); > when.tm.hour += 1; /* result is in the range [1,24] */ > deadline = mktime(when); > > printf("Loop will finish: %s\n", asctime(when)); > while (difftime(time(0), deadline)>0) whatever(); > > The semantics of mktime guarantees that the addition to the tm_hour > field produces the correct result even when the new value of tm_hour > is 24, i.e., a value outside the range ever returned by a library > funciton in a struct tm object. > > One of the reasons for adding this function is to replace the > capability to do such arithmetic which is lost when a programmer > cannot depend on time_t being an integral multiple of some known time > unit. Ignore the grammar and coding errors for the moment. The big problem (to me) is that code like the above is not a "paradigm for continuing some loop for an hour"--you may end up continuing the loop for two hours (or for no time at all) if the loop is executed just before a daylight saving time transition. If the *only* reason for adding mktime was "to replace the capability to do. . .arithmetic [on time_t type values]," I'd advocate simply throwing mktime overboard and specifying a function such as time_t addsec(time_t base, long seconds) to allow the arithmetic to be done. But the Rationale says that allowing such arithmetic is just "[o]ne of the reasons." And now, the promised questions: 1. What other reasons can folks think of for adding mktime to the standard? 2. Would it be wise to add a function to the standard to permit arithmetic on time_t type values? 3. If an arithmetic-permitting function was available, would it be wise to eliminate the ability to handle tm structures containing "out of bounds" values from mktime? -- UUCP: ..decvax!seismo!elsie!ado ARPA: elsie!ado(a)seismo.ARPA DEC, VAX, Elsie & Ado are Digital, Borden & Ampex trademarks.

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New time zone bundles available
by Arthur David Olson Jan. 3, 1987

Jan. 3, 1987

New shell archives of time zone programs and data are available; folks who are interested in getting copies should send me a note. In the new bundles I've switched from having a int settz(char * zonename) function to set the time zone to having two functions: void tzset() which sets the time zone according to the environment variable TZ, and void tzsetwall() which sets the time zone to the best available approximation to local wall clock time, regardless of what's in TZ. While several folks on the mailing list have expressed a preference for settz over tzset/tzsetwall, I can't help but believe that system vendors (who'll eventually have to bless any new standard) would favor using tzset to increase forward portability. (I'd appreciate comments on this point--unofficial and of the record, if need be--from those of you who work for system vendors.) The new bundles also include source (though, as yet, no documentation) for the POSIX mktime function; this source is a merger of work done by Guy Harris and work done by Robert Elz. I hope to submit a new time zone package to mod.sources fairly shortly. Two upcoming dates motivate this: the date on which DST rule changes take effect in the US (April 5), and an upcoming P1003 meeting in March. The more you good folks can work things over before they go off to mod.sources, the happier I'll be. -- UUCP: ..decvax!seismo!elsie!ado ARPA: elsie!ado(a)seismo.ARPA DEC, VAX, Elsie & Ado are Digital, Borden & Ampex trademarks.

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Re: Parsing /etc/TIMEZONE
by seismo!hplabs!hpfcla!hpfclj!hpfcdg!rgt Dec. 21, 1986

Dec. 21, 1986

One thing that bothers me about the new parsing routine for /etc/TIMEZONE (I can't say at this point whether it is different from the old one) i that fgets calls malloc(). One of the uses which I have for ctime strictly forbids the use of either malloc() or the standard I/O libraries. Ron Tolley

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Re: Default timezone
by seismo!sun!guy Dec. 20, 1986

Dec. 20, 1986

I mean local wallclock time (conversion rules out of an instruction would be a little wierd... but this processor has a "load.time" instruction, or something like that, which returns local wallclock time, 64 bits, high precision, and lasts forever...) By "local wallclock time", do you mean local wallclock time (DST and all), or just the number that it was originally set to at time T plus the number of clock ticks since time T? If it's the latter, it could just as easily keep "UNIX time". The time system call (I can't remember now if its in the kernel or not or if time(2) just does this in each user's code) gets that value and then applies basically the reverse transformation of localtime() to get GMT to return to the user... My point was that the method of discovering what local wall clock time is shouldn't be constrained at all in the standard. But in most cases you get wall clock time by doing "time" or "gettimeofday" and feeding that to "localtime"; if you do that on this machine, and it's OS is really UNIX-compatible, the fact that the hardware keeps some flavor of local time is irrelevant. The application would get "UNIX time", and then call "localtime" to convert that back to local time in some time zone. The standard *already* constrains the way you discover what local wall clock time is; you get "UNIX time" from "time" and feed it to "localtime". Hardware that keeps "wall clock time" is, in a POSIX system, an obstacle to overcome (you have to convert "wall clock time" to "UNIX time") rather than something that makes certain operations easier. If you really don't want the standard to constrain how you get local wall clock time, you'd have to add a function like "localtime" that takes *no* arguments, and returns the broken-down value of the wall clock time, or add a function like "time" that returns wall clock time as a "time_t" and use "gmttime" or something if you want to break that down. The S5R3.1 scheme does handle this, by having "init" put the time zone in its environment. No, that only gets it to the children of init, and their children, until someone types "env - sh" to their shell... How is typing env - sh to your shell different from typing TZ=Europe/Albania; export TZ to your shell? In either case, the user is explicitly reaming out their environment. Are you saying that even if a user reams out their environment, programs they run should get local time by default? I neither agree nor disagree with this; I haven't seen what I consider to be a good argument either way. Actually, "settz()" could be implemented in the S5R3.1 scheme. Yes, the problem is whether AT&T will do it or not. Since some way of permitting a program to get all time conversions to be done with the "official" local rules is required, the standard must include something like that. The S5R3.1 scheme already has a way of doing it; you open "/etc/TIMEZONE", read it, and "putenv" the value in there. However, 1) you don't want to have to write the same idiom 500 times for 500 programs, and 2) you want the standard to have a way of requesting "official" local time without having to know so many details of how the rules are specified, so that the standard doesn't have to require you to use TZ and all the baggage (i.e., the ability for the user to choose which rules they want) that this drags in. So you want some way of saying "please give me local rules", rather than some way of getting the "name" of the local rules and some way of saying "please give me this set of rules". "settz((char *)NULL)" seems fine to me. (The behavior when "settz" is passed a string rather than a null pointer could be left up to the implementation, in case any implementor might object to a fully general scheme.) If the standard does include that, AT&T could implement it, and would have to if they wanted to sell to somebody who required POSIX compliance.

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Re: Default timezone
by seismo!sun!guy Dec. 20, 1986

Dec. 20, 1986

While I agree that setting TZ in the environment is best used for a user modification of the default, you have to admit that an implementation that read /etc/timezone every time you did an ls -l or called ctime for logging would be awfully slow. Not really. I tested Arthur Olson's code, which doesn't read "/etc/TIMEZONE" but *does* read a file of rules, with a program that just converted the current time using "ctime". The difference in time was negligible. It probably took more time to load the test program(s) than it took to read the files - and this was a version that read files that were written with a machine-independent byte order; it wasn't just doing one big "read". Even using Sun XDR, it wasn't a major difference. (Also, remember that a program won't read "/etc/timezone", or the setup files, *every* time it calls "ctime"; once is enough. Yes, this means you have to restart long-running programs if the rules change, but that shouldn't happen too frequently.) This is why I liked the ftime system call notion; it's impossible to forge, easy to default, easy to configure, and fast. Unfortunately, it's also incomplete, unless you extend "ftime" to return a table of DST on/off times. Recompiling every program that uses "ctime" because the U.S. Congress extended DST is a pain; it's worse in countries that seem to change the rules on a yearly basis....

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Re: Default timezone
by seismo!cbpavo.MIS.OH.ATT.COM!mark Dec. 19, 1986

Dec. 19, 1986

While I agree that setting TZ in the environment is best used for a user modification of the default, you have to admit that an implementation that read /etc/timezone every time you did an ls -l or called ctime for logging would be awfully slow. This is why I liked the ftime system call notion; it's impossible to forge, easy to default, easy to configure, and fast. Mark

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Re: Default timezone
by seismo!sun!guy Dec. 18, 1986

Dec. 18, 1986

> From: seismo!cuuxb!dlm (Dennis L. Mumaugh) > Subject: Default timezone > Our reasoning is that in the majority of cases this is an error > by the system administrator in setting or exporting the TZ > variable and it is necessary for someone to notice the error. As > it stands most developers/porters set the TZ to their local time > zone (who is it that uses PST?!) and then never notice that their > product has problems. What "problems" does setting TZ to the local time zone (which *IS* PST here where we do *our* development) disguise, and why does defaulting to GMT if TZ is missing or empty reveal them? > Using wall clock as default merely propogates this development > problem and results in products having problems when they are > used in other environments. So does setting TZ in "/etc/init". The only way to flush these problems out before the product is released is to *deliberately* run it with timezones other than the developers' zone. > From: seismo!cuuxb!dlm (Dennis L. Mumaugh) > Subject: Re: Parsing /etc/TIMEZONE > I do like the idea of a single place for establishing the TZ for > a system. It makes it very easy for new administrators. Olson's scheme has this, as well; you just use the "-l" flag when running "tic". It causes a link name "localtime" to be made to the time zone specified as the argument to "-l". > Another problem with a routine to read and parse /etc/TIMEZONE is > what does one do for non-ATT shells (c shell and others) the > setting of TZ in the environment differs. That's why having > /etc/init set things up is good. That guarentees a "standard > environment". It is true someone could diddle with it, but I am > more concerned with naive users than others. You're going to need a routine to read and parse "/etc/TIMEZONE" anyway. "uucico" will have to use it; it does NOT want to use "TZ" to indicate what the local time zone is, precisely *because* somebody could diddle with it. > From: Robert Elz <seismo!munnari!kre> > Subject: Re: Default timezone > Date: Wed, 17 Dec 86 11:09:09 cst > From: cuuxb!dlm (Dennis L. Mumaugh) > Message-ID: <8612171709.AA08584@cuuxb> > The issue here really, is how does a programmer exercise his > "conscious choice" to get wall clock time? > If you wanted to define /etc/TIMEZONE or something as a > file containing the right TZ string, so that programs that > need wall clock time would be required to go read (and parse) > that file, then that would be a solution. But you had better > be prepared to define the format and name of that file, > and stick to it forever more. You should also be prepared > to go and find all the standard processes that need this > (like uucico, and others) and insert the relevant code. Actually, any scheme would require you to find all the processes that understand this, and insert some code to ensure that the machine's time zone is used. > Personally, I prefer a slightly simpler interface. That > doesn't preclude putting the information in a file, of > whatever format you prefer, but it should be the time > functions that read the file, not the user's code. Absolutely. This could be done in the S5R3.1 scheme, by providing a function to read "/etc/TIMEZONE". You could even call this function "settz()" (see below). > This also allows for more implementor flexibility, she > has lots of choices she can make as to how to obtain > the local wallclock time - from a simple ascii file, > from a precompiled binary file, from a network information > server, from the kernel via a system call, and I assume > lots of others. I can even imagine processors where local > time might be available as an instruction in the instruction > set (I can imagine such processors because we have one...) Do you mean "local wallclock time" or "local conversion rules"? With current UNIX interfaces, and with all the proposals that have popped up so far, all you could do is get the latter and use them to convert the result of "time()" to local wallclock time. > Certainly having local wall clock time be the default > when TZ is not set is a very sensible way to do things, > as it means that the system administrator doesn't need > to set up TZ for every process in the system, The S5R3.1 scheme does handle this, by having "init" put the time zone in its environment. > Most of the people on this list seem to feel that the > right way for a program to choose wallclock time explicitly, > is to putenv() TZ to a null value. Personally, I still think > settz() is a cleaner interface. Actually, "settz()" could be implemented in the S5R3.1 scheme. Assume that the time conversion code takes some string and uses it to set up the conversion rules (details deliberately left unspecified). "tzset" would use the value of 'getenv("TZ")' for this string. "settz" would do the following: if its argument is a null pointer, read "/etc/TIMEZONE" and use the string located therein; if its argument is a pointer to a null string, use GMT; otherwise, use its argument as the string in question. There is one issue here; should a process only be able to affect how its UNIX time <-> local time conversions are done, or should it also be able to affect any children it spawns? The former is effected by "settz"; as it stands, the later requires you to change TZ, either by eliminating it, nulling it out, or setting it from "/etc/TIMEZONE". "uucico" would probably want to affect its children; some other program might not. I do have another question; are we discussing UNIX or POSIX here? If the latter, should POSIX require that anything be supported other than local machine time? It may be convenient to permit users to specify other time zones, but user convenience is somewhat outside the scope of 1003.1, at least. POSIX should not *preclude* a user overriding the default time zone, and as such must provide a way for a process to override the user's override and get the default timezone; it probably must also provide a way to force its children to use the default time zone, as well. For this, all you need is a function that says "set time zone to the default"; the full generality of "settz" isn't necessary. You could have POSIX just define the behavior of "settz(char *)NULL)" and leave the rest up to the implementation. POSIX must also require that an application must, by default, get some "reasonable" time zone information, which would either be the system default time zone or, if supported, a user-specified time zone. It must do so in such a way that no application need explicitly set up anything (such as snarfing the right value of TZ from "/etc/TIMEZONE") in order to get this information, and in such a way that no application need be run in a special environment (i.e., saying "the way you set it is to put it in "/etc/profile" is out). Olson's scheme provides most of what is needed from the POSIX standpoint. "settz" could be added to the S5R3.1 scheme, and it would be equivalent (from the program's standpoint) to Olson's scheme. Both Olson's scheme, and the S5R3.1 scheme with "settz" added, would provide a way for a program to explicitly choose a particular time zone (and it would work in the same fashion in both schemes), and would use the value of TZ if one was provided. Both schemes make it simple for an administrator to set the default timezone; in Olson's scheme, you just run "zic" with the "-l" flag specifying the default zone, or make the link yourself, and in the S5R3.1 scheme you just set the "/etc/TIMEZONE" file. Neither scheme directly addresses the question of "how do you teach your children about the time zone you selected"? One way to do this would be to use "putenv"; however, that exposes various implementation details that really should not be exposed. Perhaps there should be a function like "settz" that propagates that change to children.

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