2 This file is in the public domain, so clarified as of
3 2009-05-17 by Arthur David Olson.
7 Time and date functions
8 Scope of the tz database
9 Names of time zone rule files
10 Time zone abbreviations
12 Time and time zones on Mars
14 ----- Time and date functions -----
16 These time and date functions are upwards compatible with POSIX,
17 an international standard for UNIX-like systems.
18 As of this writing, the current edition of POSIX is:
20 Standard for Information technology
21 -- Portable Operating System Interface (POSIX (R))
23 IEEE Std 1003.1, 2004 Edition
24 <http://www.opengroup.org/online-pubs?DOC=7999959899>
25 <http://www.opengroup.org/pubs/catalog/t041.htm>
27 POSIX has the following properties and limitations.
29 * In POSIX, time display in a process is controlled by the
30 environment variable TZ. Unfortunately, the POSIX TZ string takes
31 a form that is hard to describe and is error-prone in practice.
32 Also, POSIX TZ strings can't deal with other (for example, Israeli)
33 daylight saving time rules, or situations where more than two
34 time zone abbreviations are used in an area.
36 The POSIX TZ string takes the following form:
38 stdoffset[dst[offset],date[/time],date[/time]]
43 are 3 or more characters specifying the standard
44 and daylight saving time (DST) zone names.
45 Starting with POSIX.1-2001, std and dst may also be
46 in a quoted form like "<UTC+10>"; this allows
47 "+" and "-" in the names.
49 is of the form `[-]hh:[mm[:ss]]' and specifies the
50 offset west of UTC. The default DST offset is one hour
51 ahead of standard time.
52 date[/time],date[/time]
53 specifies the beginning and end of DST. If this is absent,
54 the system supplies its own rules for DST, and these can
55 differ from year to year; typically US DST rules are used.
57 takes the form `hh:[mm[:ss]]' and defaults to 02:00.
59 takes one of the following forms:
61 origin-1 day number not counting February 29
63 origin-0 day number counting February 29 if present
64 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
65 for the dth day of week n of month m of the year,
66 where week 1 is the first week in which day d appears,
67 and `5' stands for the last week in which day d appears
68 (which may be either the 4th or 5th week).
70 Here is an example POSIX TZ string, for US Pacific time using rules
71 appropriate from 1987 through 2006:
73 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
75 This POSIX TZ string is hard to remember, and mishandles time stamps
76 before 1987 and after 2006. With this package you can use this
79 TZ='America/Los_Angeles'
81 * POSIX does not define the exact meaning of TZ values like "EST5EDT".
82 Typically the current US DST rules are used to interpret such values,
83 but this means that the US DST rules are compiled into each program
84 that does time conversion. This means that when US time conversion
85 rules change (as in the United States in 1987), all programs that
86 do time conversion must be recompiled to ensure proper results.
88 * In POSIX, there's no tamper-proof way for a process to learn the
89 system's best idea of local wall clock. (This is important for
90 applications that an administrator wants used only at certain times--
91 without regard to whether the user has fiddled the "TZ" environment
92 variable. While an administrator can "do everything in UTC" to get
93 around the problem, doing so is inconvenient and precludes handling
94 daylight saving time shifts--as might be required to limit phone
95 calls to off-peak hours.)
97 * POSIX requires that systems ignore leap seconds.
99 These are the extensions that have been made to the POSIX functions:
101 * The "TZ" environment variable is used in generating the name of a file
102 from which time zone information is read (or is interpreted a la
103 POSIX); "TZ" is no longer constrained to be a three-letter time zone
104 name followed by a number of hours and an optional three-letter
105 daylight time zone name. The daylight saving time rules to be used
106 for a particular time zone are encoded in the time zone file;
107 the format of the file allows U.S., Australian, and other rules to be
108 encoded, and allows for situations where more than two time zone
109 abbreviations are used.
111 It was recognized that allowing the "TZ" environment variable to
112 take on values such as "America/New_York" might cause "old" programs
113 (that expect "TZ" to have a certain form) to operate incorrectly;
114 consideration was given to using some other environment variable
115 (for example, "TIMEZONE") to hold the string used to generate the
116 time zone information file name. In the end, however, it was decided
117 to continue using "TZ": it is widely used for time zone purposes;
118 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
119 and systems where "new" forms of "TZ" might cause problems can simply
120 use TZ values such as "EST5EDT" which can be used both by
121 "new" programs (a la POSIX) and "old" programs (as zone names and
124 * To handle places where more than two time zone abbreviations are used,
125 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
126 (where "tmp" is the value the function returns) to the time zone
127 abbreviation to be used. This differs from POSIX, where the elements
128 of tzname are only changed as a result of calls to tzset.
130 * Since the "TZ" environment variable can now be used to control time
131 conversion, the "daylight" and "timezone" variables are no longer
132 needed. (These variables are defined and set by "tzset"; however, their
133 values will not be used by "localtime.")
135 * The "localtime" function has been set up to deliver correct results
136 for near-minimum or near-maximum time_t values. (A comment in the
137 source code tells how to get compatibly wrong results).
139 * A function "tzsetwall" has been added to arrange for the system's
140 best approximation to local wall clock time to be delivered by
141 subsequent calls to "localtime." Source code for portable
142 applications that "must" run on local wall clock time should call
143 "tzsetwall();" if such code is moved to "old" systems that don't
144 provide tzsetwall, you won't be able to generate an executable program.
145 (These time zone functions also arrange for local wall clock time to be
146 used if tzset is called--directly or indirectly--and there's no "TZ"
147 environment variable; portable applications should not, however, rely
148 on this behavior since it's not the way SVR2 systems behave.)
150 * These functions can account for leap seconds, thanks to Bradley White.
152 Points of interest to folks with other systems:
154 * This package is already part of many POSIX-compliant hosts,
155 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
156 On such hosts, the primary use of this package
157 is to update obsolete time zone rule tables.
158 To do this, you may need to compile the time zone compiler
159 `zic' supplied with this package instead of using the system `zic',
160 since the format of zic's input changed slightly in late 1994,
161 and many vendors still do not support the new input format.
163 * The UNIX Version 7 "timezone" function is not present in this package;
164 it's impossible to reliably map timezone's arguments (a "minutes west
165 of GMT" value and a "daylight saving time in effect" flag) to a
166 time zone abbreviation, and we refuse to guess.
167 Programs that in the past used the timezone function may now examine
168 tzname[localtime(&clock)->tm_isdst] to learn the correct time
169 zone abbreviation to use. Alternatively, use
170 localtime(&clock)->tm_zone if this has been enabled.
172 * The 4.2BSD gettimeofday function is not used in this package.
173 This formerly let users obtain the current UTC offset and DST flag,
174 but this functionality was removed in later versions of BSD.
176 * In SVR2, time conversion fails for near-minimum or near-maximum
177 time_t values when doing conversions for places that don't use UTC.
178 This package takes care to do these conversions correctly.
180 The functions that are conditionally compiled if STD_INSPIRED is defined
181 should, at this point, be looked on primarily as food for thought. They are
182 not in any sense "standard compatible"--some are not, in fact, specified in
183 *any* standard. They do, however, represent responses of various authors to
184 standardization proposals.
186 Other time conversion proposals, in particular the one developed by folks at
187 Hewlett Packard, offer a wider selection of functions that provide capabilities
188 beyond those provided here. The absence of such functions from this package
189 is not meant to discourage the development, standardization, or use of such
190 functions. Rather, their absence reflects the decision to make this package
191 contain valid extensions to POSIX, to ensure its broad acceptability. If
192 more powerful time conversion functions can be standardized, so much the
196 ----- Scope of the tz database -----
198 The tz database attempts to record the history and predicted future of
199 all computer-based clocks that track civil time. To represent this
200 data, the world is partitioned into regions whose clocks all agree
201 about time stamps that occur after the somewhat-arbitrary cutoff point
202 of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region,
203 the database records all known clock transitions, and labels the region
204 with a notable location.
206 Clock transitions before 1970 are recorded for each such location,
207 because most POSIX-compatible systems support negative time stamps and
208 could misbehave if data were omitted for pre-1970 transitions.
209 However, the database is not designed for and does not suffice for
210 applications requiring accurate handling of all past times everywhere,
211 as it would take far too much effort and guesswork to record all
212 details of pre-1970 civil timekeeping.
214 As noted in the README file, the tz database is not authoritative
215 (particularly not for pre-1970 time stamps), and it surely has errors.
216 Corrections are welcome and encouraged. Users requiring authoritative
217 data should consult national standards bodies and the references cited
218 in the database's comments.
221 ----- Names of time zone rule files -----
223 The time zone rule file naming conventions attempt to strike a balance
224 among the following goals:
226 * Uniquely identify every national region where clocks have all
227 agreed since 1970. This is essential for the intended use: static
228 clocks keeping local civil time.
230 * Indicate to humans as to where that region is. This simplifes use.
232 * Be robust in the presence of political changes. This reduces the
233 number of updates and backward-compatibility hacks. For example,
234 names of countries are ordinarily not used, to avoid
235 incompatibilities when countries change their name
236 (e.g. Zaire->Congo) or when locations change countries
237 (e.g. Hong Kong from UK colony to China).
239 * Be portable to a wide variety of implementations.
240 This promotes use of the technology.
242 * Use a consistent naming convention over the entire world.
243 This simplifies both use and maintenance.
245 This naming convention is not intended for use by inexperienced users
246 to select TZ values by themselves (though they can of course examine
247 and reuse existing settings). Distributors should provide
248 documentation and/or a simple selection interface that explains the
249 names; see the 'tzselect' program supplied with this distribution for
252 Names normally have the form AREA/LOCATION, where AREA is the name
253 of a continent or ocean, and LOCATION is the name of a specific
254 location within that region. North and South America share the same
255 area, `America'. Typical names are `Africa/Cairo', `America/New_York',
256 and `Pacific/Honolulu'.
258 Here are the general rules used for choosing location names,
259 in decreasing order of importance:
261 Use only valid POSIX file name components (i.e., the parts of
262 names other than `/'). Within a file name component,
263 use only ASCII letters, `.', `-' and `_'. Do not use
264 digits, as that might create an ambiguity with POSIX
265 TZ strings. A file name component must not exceed 14
266 characters or start with `-'. E.g., prefer `Brunei'
267 to `Bandar_Seri_Begawan'.
268 Include at least one location per time zone rule set per country.
269 One such location is enough. Use ISO 3166 (see the file
270 iso3166.tab) to help decide whether something is a country.
271 However, uninhabited ISO 3166 regions like Bouvet Island
272 do not need locations, since local time is not defined there.
273 If all the clocks in a country's region have agreed since 1970,
274 don't bother to include more than one location
275 even if subregions' clocks disagreed before 1970.
276 Otherwise these tables would become annoyingly large.
277 If a name is ambiguous, use a less ambiguous alternative;
278 e.g. many cities are named San Jose and Georgetown, so
279 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'.
280 Keep locations compact. Use cities or small islands, not countries
281 or regions, so that any future time zone changes do not split
282 locations into different time zones. E.g. prefer `Paris'
283 to `France', since France has had multiple time zones.
284 Use mainstream English spelling, e.g. prefer `Rome' to `Roma', and
285 prefer `Athens' to the true name (which uses Greek letters).
286 The POSIX file name restrictions encourage this rule.
287 Use the most populous among locations in a country's time zone,
288 e.g. prefer `Shanghai' to `Beijing'. Among locations with
289 similar populations, pick the best-known location,
290 e.g. prefer `Rome' to `Milan'.
291 Use the singular form, e.g. prefer `Canary' to `Canaries'.
292 Omit common suffixes like `_Islands' and `_City', unless that
293 would lead to ambiguity. E.g. prefer `Cayman' to
294 `Cayman_Islands' and `Guatemala' to `Guatemala_City',
295 but prefer `Mexico_City' to `Mexico' because the country
296 of Mexico has several time zones.
297 Use `_' to represent a space.
298 Omit `.' from abbreviations in names, e.g. prefer `St_Helena'
300 Do not change established names if they only marginally
301 violate the above rules. For example, don't change
302 the existing name `Rome' to `Milan' merely because
303 Milan's population has grown to be somewhat greater
305 If a name is changed, put its old spelling in the `backward' file.
307 The file `zone.tab' lists the geographical locations used to name
308 time zone rule files. It is intended to be an exhaustive list
309 of canonical names for geographic regions.
311 Older versions of this package used a different naming scheme,
312 and these older names are still supported.
313 See the file `backward' for most of these older names
314 (e.g. `US/Eastern' instead of `America/New_York').
315 The other old-fashioned names still supported are
316 `WET', `CET', `MET', and `EET' (see the file `europe').
319 ----- Time zone abbreviations -----
321 When this package is installed, it generates time zone abbreviations
322 like `EST' to be compatible with human tradition and POSIX.
323 Here are the general rules used for choosing time zone abbreviations,
324 in decreasing order of importance:
326 Use abbreviations that consist of three or more ASCII letters.
327 Previous editions of this database also used characters like
328 ' ' and '?', but these characters have a special meaning to
329 the shell and cause commands like
331 to have unexpected effects.
332 Previous editions of this rule required upper-case letters,
333 but the Congressman who introduced Chamorro Standard Time
334 preferred "ChST", so the rule has been relaxed.
336 This rule guarantees that all abbreviations could have
337 been specified by a POSIX TZ string. POSIX
338 requires at least three characters for an
339 abbreviation. POSIX through 2000 says that an abbreviation
340 cannot start with ':', and cannot contain ',', '-',
341 '+', NUL, or a digit. POSIX from 2001 on changes this
342 rule to say that an abbreviation can contain only '-', '+',
343 and alphanumeric characters from the portable character set
344 in the current locale. To be portable to both sets of
345 rules, an abbreviation must therefore use only ASCII
348 Use abbreviations that are in common use among English-speakers,
349 e.g. `EST' for Eastern Standard Time in North America.
350 We assume that applications translate them to other languages
351 as part of the normal localization process; for example,
352 a French application might translate `EST' to `HNE'.
354 For zones whose times are taken from a city's longitude, use the
355 traditional xMT notation, e.g. `PMT' for Paris Mean Time.
356 The only name like this in current use is `GMT'.
358 If there is no common English abbreviation, abbreviate the English
359 translation of the usual phrase used by native speakers.
360 If this is not available or is a phrase mentioning the country
361 (e.g. ``Cape Verde Time''), then:
363 When a country has a single or principal time zone region,
364 append `T' to the country's ISO code, e.g. `CVT' for
365 Cape Verde Time. For summer time append `ST';
366 for double summer time append `DST'; etc.
367 When a country has multiple time zones, take the first three
368 letters of an English place name identifying each zone
369 and then append `T', `ST', etc. as before;
370 e.g. `VLAST' for VLAdivostok Summer Time.
372 Use UTC (with time zone abbreviation "zzz") for locations while
373 uninhabited. The "zzz" mnemonic is that these locations are,
374 in some sense, asleep.
376 Application writers should note that these abbreviations are ambiguous
377 in practice: e.g. `EST' has a different meaning in Australia than
378 it does in the United States. In new applications, it's often better
379 to use numeric UTC offsets like `-0500' instead of time zone
380 abbreviations like `EST'; this avoids the ambiguity.
383 ----- Calendrical issues -----
385 Calendrical issues are a bit out of scope for a time zone database,
386 but they indicate the sort of problems that we would run into if we
387 extended the time zone database further into the past. An excellent
388 resource in this area is Nachum Dershowitz and Edward M. Reingold,
389 <a href="http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/">
390 Calendrical Calculations: Third Edition
391 </a>, Cambridge University Press (2008). Other information and
392 sources are given below. They sometimes disagree.
397 Gregorian calendar adopted 1582-12-20.
398 French Revolutionary calendar used 1793-11-24 through 1805-12-31,
399 and (in Paris only) 1871-05-06 through 1871-05-23.
404 From Chris Carrier (1996-12-02):
405 On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar''
406 with 30-day months plus 5 holidays, with a 5-day week.
407 On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
408 Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
409 reverted to the 7-day week. With the 6-day week the usual days
410 off were the 6th, 12th, 18th, 24th and 30th of the month.
411 (Source: Evitiar Zerubavel, _The Seven Day Circle_)
414 Mark Brader reported a similar story in "The Book of Calendars", edited
415 by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But:
417 From: Petteri Sulonen (via Usenet)
418 Date: 14 Jan 1999 00:00:00 GMT
421 If your source is correct, how come documents between 1929 -- 1940 were
422 still dated using the conventional, Gregorian calendar?
424 I can post a scan of a document dated December 1, 1934, signed by
425 Yenukidze, the secretary, on behalf of Kalinin, the President of the
426 Executive Committee of the Supreme Soviet, if you like.
433 <a href="news:1996Jul6.012937.29190@sq.com">
434 Subject: Re: Gregorian reform -- a part of locale?
438 In 1700, Denmark made the transition from Julian to Gregorian. Sweden
439 decided to *start* a transition in 1700 as well, but rather than have one of
440 those unsightly calendar gaps :-), they simply decreed that the next leap
441 year after 1696 would be in 1744 -- putting the whole country on a calendar
442 different from both Julian and Gregorian for a period of 40 years.
444 However, in 1704 something went wrong and the plan was not carried through;
445 they did, after all, have a leap year that year. And one in 1708. In 1712
446 they gave it up and went back to Julian, putting 30 days in February that
449 Then in 1753, Sweden made the transition to Gregorian in the usual manner,
450 getting there only 13 years behind the original schedule.
452 (A previous posting of this story was challenged, and Swedish readers
453 produced the following references to support it: "Tiderakning och historia"
454 by Natanael Beckman (1924) and "Tid, en bok om tiderakning och
455 kalendervasen" by Lars-Olof Lode'n (no date was given).)
460 From: "Michael Palmer" [with one obvious typo fixed]
461 Subject: Re: Gregorian Calendar (was Re: Another FHC related question
462 Newsgroups: soc.genealogy.german
463 Date: Tue, 9 Feb 1999 02:32:48 -800
466 The following is a(n incomplete) listing, arranged chronologically, of
467 European states, with the date they converted from the Julian to the
470 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
471 Catholics and Danzig only)
472 09/20 Dec 1582 - France, Lorraine
475 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
476 10/21 Feb 1583 - bishopric of Liege (L"uttich)
477 13/24 Feb 1583 - bishopric of Augsburg
478 04/15 Oct 1583 - electorate of Trier
479 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
481 13/24 Oct 1583 - Austrian Oberelsass and Breisgau
482 20/31 Oct 1583 - bishopric of Basel
483 02/13 Nov 1583 - duchy of J"ulich-Berg
484 02/13 Nov 1583 - electorate and city of K"oln
485 04/15 Nov 1583 - bishopric of W"urzburg
486 11/22 Nov 1583 - electorate of Mainz
487 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
488 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve
489 14/25 Dec 1583 - Steiermark
491 06/17 Jan 1584 - Austria and Bohemia
492 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn
493 12/23 Jan 1584 - Silesia and the Lausitz
495 02 Feb 1584 - Hungary (legally on 21 Oct 1587)
496 Jun 1584 - Unterwalden
497 01/12 Jul 1584 - duchy of Westfalen
499 16/27 Jun 1585 - bishopric of Paderborn
501 14/25 Dec 1590 - Transylvania
504 02 Sep 1612 - duchy of Prussia
506 13/24 Dec 1614 - Pfalz-Neuburg
508 1617 - duchy of Kurland (reverted to the Julian calendar in
511 1624 - bishopric of Osnabr"uck
513 1630 - bishopric of Minden
515 15/26 Mar 1631 - bishopric of Hildesheim
519 05/16 Feb 1682 - city of Strassburg
522 01 Mar 1700 - Protestant Germany (including Swedish possessions in
523 Germany), Denmark, Norway
525 12 Jul 1700 - Gelderland, Zutphen
527 12 Dec 1700 - Utrecht, Overijssel
530 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva,
531 Turgau, and Schaffhausen
533 1724 - Glarus, Appenzell, and the city of St. Gallen
535 01 Jan 1750 - Pisa and Florence
537 02/14 Sep 1752 - Great Britain
542 1760-1812 - Graub"unden
544 The Russian empire (including Finland and the Baltic states) did not
545 convert to the Gregorian calendar until the Soviet revolution of 1917.
547 Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
548 Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
549 (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
552 ----- Time and time zones on Mars -----
554 Some people have adjusted their work schedules to fit Mars time.
555 Dozens of special Mars watches were built for Jet Propulsion
556 Laboratory workers who kept Mars time during the Mars Exploration
557 Rovers mission (2004). These timepieces look like normal Seikos and
558 Citizens but use Mars seconds rather than terrestrial seconds.
560 A Mars solar day is called a "sol" and has a mean period equal to
561 about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
562 divided into a conventional 24-hour clock, so each Mars second equals
563 about 1.02749125 terrestrial seconds.
565 The prime meridian of Mars goes through the center of the crater
566 Airy-0, named in honor of the British astronomer who built the
567 Greenwich telescope that defines Earth's prime meridian. Mean solar
568 time on the Mars prime meridian is called Mars Coordinated Time (MTC).
570 Each landed mission on Mars has adopted a different reference for
571 solar time keeping, so there is no real standard for Mars time zones.
572 For example, the Mars Exploration Rover project (2004) defined two
573 time zones "Local Solar Time A" and "Local Solar Time B" for its two
574 missions, each zone designed so that its time equals local true solar
575 time at approximately the middle of the nominal mission. Such a "time
576 zone" is not particularly suited for any application other than the
579 Many calendars have been proposed for Mars, but none have achieved
580 wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
581 sequential count of Mars solar days elapsed since about 1873-12-29
584 The tz database does not currently support Mars time, but it is
585 documented here in the hopes that support will be added eventually.
589 Michael Allison and Robert Schmunk,
590 "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
591 <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-07-30).
593 Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
594 (2004-01-14), pp A1, A20-A21.