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 Names of time zone regions
9 Time zone abbreviations
11 Time and time zones on Mars
13 ----- Time and date functions -----
15 These time and date functions are upwards compatible with POSIX,
16 an international standard for UNIX-like systems.
17 As of this writing, the current edition of POSIX is:
19 Standard for Information technology
20 -- Portable Operating System Interface (POSIX (R))
22 IEEE Std 1003.1, 2004 Edition
23 <http://www.opengroup.org/online-pubs?DOC=7999959899>
24 <http://www.opengroup.org/pubs/catalog/t041.htm>
26 POSIX has the following properties and limitations.
28 * In POSIX, time display in a process is controlled by the
29 environment variable TZ. Unfortunately, the POSIX TZ string takes
30 a form that is hard to describe and is error-prone in practice.
31 Also, POSIX TZ strings can't deal with other (for example, Israeli)
32 daylight saving time rules, or situations where more than two
33 time zone abbreviations are used in an area.
35 The POSIX TZ string takes the following form:
37 stdoffset[dst[offset],date[/time],date[/time]]
42 are 3 or more characters specifying the standard
43 and daylight saving time (DST) zone names.
44 Starting with POSIX.1-2001, std and dst may also be
45 in a quoted form like "<UTC+10>"; this allows
46 "+" and "-" in the names.
48 is of the form `[-]hh:[mm[:ss]]' and specifies the
49 offset west of UTC. The default DST offset is one hour
50 ahead of standard time.
51 date[/time],date[/time]
52 specifies the beginning and end of DST. If this is absent,
53 the system supplies its own rules for DST, and these can
54 differ from year to year; typically US DST rules are used.
56 takes the form `hh:[mm[:ss]]' and defaults to 02:00.
58 takes one of the following forms:
60 origin-1 day number not counting February 29
62 origin-0 day number counting February 29 if present
63 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
64 for the dth day of week n of month m of the year,
65 where week 1 is the first week in which day d appears,
66 and `5' stands for the last week in which day d appears
67 (which may be either the 4th or 5th week).
69 Here is an example POSIX TZ string, for US Pacific time using rules
70 appropriate from 1987 through 2006:
72 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
74 This POSIX TZ string is hard to remember, and mishandles time stamps
75 before 1987 and after 2006. With this package you can use this
78 TZ='America/Los_Angeles'
80 * POSIX does not define the exact meaning of TZ values like "EST5EDT".
81 Typically the current US DST rules are used to interpret such values,
82 but this means that the US DST rules are compiled into each program
83 that does time conversion. This means that when US time conversion
84 rules change (as in the United States in 1987), all programs that
85 do time conversion must be recompiled to ensure proper results.
87 * In POSIX, there's no tamper-proof way for a process to learn the
88 system's best idea of local wall clock. (This is important for
89 applications that an administrator wants used only at certain times--
90 without regard to whether the user has fiddled the "TZ" environment
91 variable. While an administrator can "do everything in UTC" to get
92 around the problem, doing so is inconvenient and precludes handling
93 daylight saving time shifts--as might be required to limit phone
94 calls to off-peak hours.)
96 * POSIX requires that systems ignore leap seconds.
98 These are the extensions that have been made to the POSIX functions:
100 * The "TZ" environment variable is used in generating the name of a file
101 from which time zone information is read (or is interpreted a la
102 POSIX); "TZ" is no longer constrained to be a three-letter time zone
103 name followed by a number of hours and an optional three-letter
104 daylight time zone name. The daylight saving time rules to be used
105 for a particular time zone are encoded in the time zone file;
106 the format of the file allows U.S., Australian, and other rules to be
107 encoded, and allows for situations where more than two time zone
108 abbreviations are used.
110 It was recognized that allowing the "TZ" environment variable to
111 take on values such as "America/New_York" might cause "old" programs
112 (that expect "TZ" to have a certain form) to operate incorrectly;
113 consideration was given to using some other environment variable
114 (for example, "TIMEZONE") to hold the string used to generate the
115 time zone information file name. In the end, however, it was decided
116 to continue using "TZ": it is widely used for time zone purposes;
117 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
118 and systems where "new" forms of "TZ" might cause problems can simply
119 use TZ values such as "EST5EDT" which can be used both by
120 "new" programs (a la POSIX) and "old" programs (as zone names and
123 * To handle places where more than two time zone abbreviations are used,
124 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
125 (where "tmp" is the value the function returns) to the time zone
126 abbreviation to be used. This differs from POSIX, where the elements
127 of tzname are only changed as a result of calls to tzset.
129 * Since the "TZ" environment variable can now be used to control time
130 conversion, the "daylight" and "timezone" variables are no longer
131 needed. (These variables are defined and set by "tzset"; however, their
132 values will not be used by "localtime.")
134 * The "localtime" function has been set up to deliver correct results
135 for near-minimum or near-maximum time_t values. (A comment in the
136 source code tells how to get compatibly wrong results).
138 * A function "tzsetwall" has been added to arrange for the system's
139 best approximation to local wall clock time to be delivered by
140 subsequent calls to "localtime." Source code for portable
141 applications that "must" run on local wall clock time should call
142 "tzsetwall();" if such code is moved to "old" systems that don't
143 provide tzsetwall, you won't be able to generate an executable program.
144 (These time zone functions also arrange for local wall clock time to be
145 used if tzset is called--directly or indirectly--and there's no "TZ"
146 environment variable; portable applications should not, however, rely
147 on this behavior since it's not the way SVR2 systems behave.)
149 * These functions can account for leap seconds, thanks to Bradley White.
151 Points of interest to folks with other systems:
153 * This package is already part of many POSIX-compliant hosts,
154 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
155 On such hosts, the primary use of this package
156 is to update obsolete time zone rule tables.
157 To do this, you may need to compile the time zone compiler
158 `zic' supplied with this package instead of using the system `zic',
159 since the format of zic's input changed slightly in late 1994,
160 and many vendors still do not support the new input format.
162 * The UNIX Version 7 "timezone" function is not present in this package;
163 it's impossible to reliably map timezone's arguments (a "minutes west
164 of GMT" value and a "daylight saving time in effect" flag) to a
165 time zone abbreviation, and we refuse to guess.
166 Programs that in the past used the timezone function may now examine
167 tzname[localtime(&clock)->tm_isdst] to learn the correct time
168 zone abbreviation to use. Alternatively, use
169 localtime(&clock)->tm_zone if this has been enabled.
171 * The 4.2BSD gettimeofday function is not used in this package.
172 This formerly let users obtain the current UTC offset and DST flag,
173 but this functionality was removed in later versions of BSD.
175 * In SVR2, time conversion fails for near-minimum or near-maximum
176 time_t values when doing conversions for places that don't use UTC.
177 This package takes care to do these conversions correctly.
179 The functions that are conditionally compiled if STD_INSPIRED is defined
180 should, at this point, be looked on primarily as food for thought. They are
181 not in any sense "standard compatible"--some are not, in fact, specified in
182 *any* standard. They do, however, represent responses of various authors to
183 standardization proposals.
185 Other time conversion proposals, in particular the one developed by folks at
186 Hewlett Packard, offer a wider selection of functions that provide capabilities
187 beyond those provided here. The absence of such functions from this package
188 is not meant to discourage the development, standardization, or use of such
189 functions. Rather, their absence reflects the decision to make this package
190 contain valid extensions to POSIX, to ensure its broad acceptability. If
191 more powerful time conversion functions can be standardized, so much the
195 ----- Names of time zone rule files -----
197 The time zone rule file naming conventions attempt to strike a balance
198 among the following goals:
200 * Uniquely identify every national region where clocks have all
201 agreed since 1970. This is essential for the intended use: static
202 clocks keeping local civil time.
204 * Indicate to humans as to where that region is. This simplifes use.
206 * Be robust in the presence of political changes. This reduces the
207 number of updates and backward-compatibility hacks. For example,
208 names of countries are ordinarily not used, to avoid
209 incompatibilities when countries change their name
210 (e.g. Zaire->Congo) or when locations change countries
211 (e.g. Hong Kong from UK colony to China).
213 * Be portable to a wide variety of implementations.
214 This promotes use of the technology.
216 * Use a consistent naming convention over the entire world.
217 This simplifies both use and maintenance.
219 This naming convention is not intended for use by inexperienced users
220 to select TZ values by themselves (though they can of course examine
221 and reuse existing settings). Distributors should provide
222 documentation and/or a simple selection interface that explains the
223 names; see the 'tzselect' program supplied with this distribution for
226 Names normally have the form AREA/LOCATION, where AREA is the name
227 of a continent or ocean, and LOCATION is the name of a specific
228 location within that region. North and South America share the same
229 area, `America'. Typical names are `Africa/Cairo', `America/New_York',
230 and `Pacific/Honolulu'.
232 Here are the general rules used for choosing location names,
233 in decreasing order of importance:
235 Use only valid POSIX file name components (i.e., the parts of
236 names other than `/'). Within a file name component,
237 use only ASCII letters, `.', `-' and `_'. Do not use
238 digits, as that might create an ambiguity with POSIX
239 TZ strings. A file name component must not exceed 14
240 characters or start with `-'. E.g., prefer `Brunei'
241 to `Bandar_Seri_Begawan'.
242 Include at least one location per time zone rule set per country.
243 One such location is enough. Use ISO 3166 (see the file
244 iso3166.tab) to help decide whether something is a country.
245 However, uninhabited ISO 3166 regions like Bouvet Island
246 do not need locations, since local time is not defined there.
247 If all the clocks in a country's region have agreed since 1970,
248 don't bother to include more than one location
249 even if subregions' clocks disagreed before 1970.
250 Otherwise these tables would become annoyingly large.
251 If a name is ambiguous, use a less ambiguous alternative;
252 e.g. many cities are named San Jose and Georgetown, so
253 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'.
254 Keep locations compact. Use cities or small islands, not countries
255 or regions, so that any future time zone changes do not split
256 locations into different time zones. E.g. prefer `Paris'
257 to `France', since France has had multiple time zones.
258 Use mainstream English spelling, e.g. prefer `Rome' to `Roma', and
259 prefer `Athens' to the true name (which uses Greek letters).
260 The POSIX file name restrictions encourage this rule.
261 Use the most populous among locations in a country's time zone,
262 e.g. prefer `Shanghai' to `Beijing'. Among locations with
263 similar populations, pick the best-known location,
264 e.g. prefer `Rome' to `Milan'.
265 Use the singular form, e.g. prefer `Canary' to `Canaries'.
266 Omit common suffixes like `_Islands' and `_City', unless that
267 would lead to ambiguity. E.g. prefer `Cayman' to
268 `Cayman_Islands' and `Guatemala' to `Guatemala_City',
269 but prefer `Mexico_City' to `Mexico' because the country
270 of Mexico has several time zones.
271 Use `_' to represent a space.
272 Omit `.' from abbreviations in names, e.g. prefer `St_Helena'
274 Do not change established names if they only marginally
275 violate the above rules. For example, don't change
276 the existing name `Rome' to `Milan' merely because
277 Milan's population has grown to be somewhat greater
279 If a name is changed, put its old spelling in the `backward' file.
281 The file `zone.tab' lists the geographical locations used to name
282 time zone rule files. It is intended to be an exhaustive list
283 of canonical names for geographic regions.
285 Older versions of this package used a different naming scheme,
286 and these older names are still supported.
287 See the file `backward' for most of these older names
288 (e.g. `US/Eastern' instead of `America/New_York').
289 The other old-fashioned names still supported are
290 `WET', `CET', `MET', and `EET' (see the file `europe').
293 ----- Time zone abbreviations -----
295 When this package is installed, it generates time zone abbreviations
296 like `EST' to be compatible with human tradition and POSIX.
297 Here are the general rules used for choosing time zone abbreviations,
298 in decreasing order of importance:
300 Use abbreviations that consist of three or more ASCII letters.
301 Previous editions of this database also used characters like
302 ' ' and '?', but these characters have a special meaning to
303 the shell and cause commands like
305 to have unexpected effects.
306 Previous editions of this rule required upper-case letters,
307 but the Congressman who introduced Chamorro Standard Time
308 preferred "ChST", so the rule has been relaxed.
310 This rule guarantees that all abbreviations could have
311 been specified by a POSIX TZ string. POSIX
312 requires at least three characters for an
313 abbreviation. POSIX through 2000 says that an abbreviation
314 cannot start with ':', and cannot contain ',', '-',
315 '+', NUL, or a digit. POSIX from 2001 on changes this
316 rule to say that an abbreviation can contain only '-', '+',
317 and alphanumeric characters from the portable character set
318 in the current locale. To be portable to both sets of
319 rules, an abbreviation must therefore use only ASCII
322 Use abbreviations that are in common use among English-speakers,
323 e.g. `EST' for Eastern Standard Time in North America.
324 We assume that applications translate them to other languages
325 as part of the normal localization process; for example,
326 a French application might translate `EST' to `HNE'.
328 For zones whose times are taken from a city's longitude, use the
329 traditional xMT notation, e.g. `PMT' for Paris Mean Time.
330 The only name like this in current use is `GMT'.
332 If there is no common English abbreviation, abbreviate the English
333 translation of the usual phrase used by native speakers.
334 If this is not available or is a phrase mentioning the country
335 (e.g. ``Cape Verde Time''), then:
337 When a country has a single or principal time zone region,
338 append `T' to the country's ISO code, e.g. `CVT' for
339 Cape Verde Time. For summer time append `ST';
340 for double summer time append `DST'; etc.
341 When a country has multiple time zones, take the first three
342 letters of an English place name identifying each zone
343 and then append `T', `ST', etc. as before;
344 e.g. `VLAST' for VLAdivostok Summer Time.
346 Use UTC (with time zone abbreviation "zzz") for locations while
347 uninhabited. The "zzz" mnemonic is that these locations are,
348 in some sense, asleep.
350 Application writers should note that these abbreviations are ambiguous
351 in practice: e.g. `EST' has a different meaning in Australia than
352 it does in the United States. In new applications, it's often better
353 to use numeric UTC offsets like `-0500' instead of time zone
354 abbreviations like `EST'; this avoids the ambiguity.
357 ----- Calendrical issues -----
359 Calendrical issues are a bit out of scope for a time zone database,
360 but they indicate the sort of problems that we would run into if we
361 extended the time zone database further into the past. An excellent
362 resource in this area is Nachum Dershowitz and Edward M. Reingold,
363 <a href="http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/">
364 Calendrical Calculations: Third Edition
365 </a>, Cambridge University Press (2008). Other information and
366 sources are given below. They sometimes disagree.
371 Gregorian calendar adopted 1582-12-20.
372 French Revolutionary calendar used 1793-11-24 through 1805-12-31,
373 and (in Paris only) 1871-05-06 through 1871-05-23.
378 From Chris Carrier (1996-12-02):
379 On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar''
380 with 30-day months plus 5 holidays, with a 5-day week.
381 On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
382 Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
383 reverted to the 7-day week. With the 6-day week the usual days
384 off were the 6th, 12th, 18th, 24th and 30th of the month.
385 (Source: Evitiar Zerubavel, _The Seven Day Circle_)
388 Mark Brader reported a similar story in "The Book of Calendars", edited
389 by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But:
391 From: Petteri Sulonen (via Usenet)
392 Date: 14 Jan 1999 00:00:00 GMT
395 If your source is correct, how come documents between 1929 -- 1940 were
396 still dated using the conventional, Gregorian calendar?
398 I can post a scan of a document dated December 1, 1934, signed by
399 Yenukidze, the secretary, on behalf of Kalinin, the President of the
400 Executive Committee of the Supreme Soviet, if you like.
407 <a href="news:1996Jul6.012937.29190@sq.com">
408 Subject: Re: Gregorian reform -- a part of locale?
412 In 1700, Denmark made the transition from Julian to Gregorian. Sweden
413 decided to *start* a transition in 1700 as well, but rather than have one of
414 those unsightly calendar gaps :-), they simply decreed that the next leap
415 year after 1696 would be in 1744 -- putting the whole country on a calendar
416 different from both Julian and Gregorian for a period of 40 years.
418 However, in 1704 something went wrong and the plan was not carried through;
419 they did, after all, have a leap year that year. And one in 1708. In 1712
420 they gave it up and went back to Julian, putting 30 days in February that
423 Then in 1753, Sweden made the transition to Gregorian in the usual manner,
424 getting there only 13 years behind the original schedule.
426 (A previous posting of this story was challenged, and Swedish readers
427 produced the following references to support it: "Tiderakning och historia"
428 by Natanael Beckman (1924) and "Tid, en bok om tiderakning och
429 kalendervasen" by Lars-Olof Lode'n (no date was given).)
434 From: "Michael Palmer" [with one obvious typo fixed]
435 Subject: Re: Gregorian Calendar (was Re: Another FHC related question
436 Newsgroups: soc.genealogy.german
437 Date: Tue, 9 Feb 1999 02:32:48 -800
440 The following is a(n incomplete) listing, arranged chronologically, of
441 European states, with the date they converted from the Julian to the
444 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
445 Catholics and Danzig only)
446 09/20 Dec 1582 - France, Lorraine
449 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
450 10/21 Feb 1583 - bishopric of Liege (L"uttich)
451 13/24 Feb 1583 - bishopric of Augsburg
452 04/15 Oct 1583 - electorate of Trier
453 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
455 13/24 Oct 1583 - Austrian Oberelsass and Breisgau
456 20/31 Oct 1583 - bishopric of Basel
457 02/13 Nov 1583 - duchy of J"ulich-Berg
458 02/13 Nov 1583 - electorate and city of K"oln
459 04/15 Nov 1583 - bishopric of W"urzburg
460 11/22 Nov 1583 - electorate of Mainz
461 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
462 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve
463 14/25 Dec 1583 - Steiermark
465 06/17 Jan 1584 - Austria and Bohemia
466 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn
467 12/23 Jan 1584 - Silesia and the Lausitz
469 02 Feb 1584 - Hungary (legally on 21 Oct 1587)
470 Jun 1584 - Unterwalden
471 01/12 Jul 1584 - duchy of Westfalen
473 16/27 Jun 1585 - bishopric of Paderborn
475 14/25 Dec 1590 - Transylvania
478 02 Sep 1612 - duchy of Prussia
480 13/24 Dec 1614 - Pfalz-Neuburg
482 1617 - duchy of Kurland (reverted to the Julian calendar in
485 1624 - bishopric of Osnabr"uck
487 1630 - bishopric of Minden
489 15/26 Mar 1631 - bishopric of Hildesheim
493 05/16 Feb 1682 - city of Strassburg
496 01 Mar 1700 - Protestant Germany (including Swedish possessions in
497 Germany), Denmark, Norway
499 12 Jul 1700 - Gelderland, Zutphen
501 12 Dec 1700 - Utrecht, Overijssel
504 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva,
505 Turgau, and Schaffhausen
507 1724 - Glarus, Appenzell, and the city of St. Gallen
509 01 Jan 1750 - Pisa and Florence
511 02/14 Sep 1752 - Great Britain
516 1760-1812 - Graub"unden
518 The Russian empire (including Finland and the Baltic states) did not
519 convert to the Gregorian calendar until the Soviet revolution of 1917.
521 Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
522 Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
523 (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
526 ----- Time and time zones on Mars -----
528 Some people have adjusted their work schedules to fit Mars time.
529 Dozens of special Mars watches were built for Jet Propulsion
530 Laboratory workers who kept Mars time during the Mars Exploration
531 Rovers mission (2004). These timepieces look like normal Seikos and
532 Citizens but use Mars seconds rather than terrestrial seconds.
534 A Mars solar day is called a "sol" and has a mean period equal to
535 about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
536 divided into a conventional 24-hour clock, so each Mars second equals
537 about 1.02749125 terrestrial seconds.
539 The prime meridian of Mars goes through the center of the crater
540 Airy-0, named in honor of the British astronomer who built the
541 Greenwich telescope that defines Earth's prime meridian. Mean solar
542 time on the Mars prime meridian is called Mars Coordinated Time (MTC).
544 Each landed mission on Mars has adopted a different reference for
545 solar time keeping, so there is no real standard for Mars time zones.
546 For example, the Mars Exploration Rover project (2004) defined two
547 time zones "Local Solar Time A" and "Local Solar Time B" for its two
548 missions, each zone designed so that its time equals local true solar
549 time at approximately the middle of the nominal mission. Such a "time
550 zone" is not particularly suited for any application other than the
553 Many calendars have been proposed for Mars, but none have achieved
554 wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
555 sequential count of Mars solar days elapsed since about 1873-12-29
558 The tz database does not currently support Mars time, but it is
559 documented here in the hopes that support will be added eventually.
563 Michael Allison and Robert Schmunk,
564 "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
565 <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-07-30).
567 Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
568 (2004-01-14), pp A1, A20-A21.