Theory revision 1.10 1 # $NetBSD: Theory,v 1.10 2011/09/04 10:10:26 christos Exp $
2 @(#)Theory 8.6
3 This file is in the public domain, so clarified as of
4 2009-05-17 by Arthur David Olson.
5
6 ----- Outline -----
7
8 Time and date functions
9 Scope of the tz database
10 Names of time zone rule files
11 Time zone abbreviations
12 Calendrical issues
13 Time and time zones on Mars
14
15 ----- Time and date functions -----
16
17 These time and date functions are upwards compatible with POSIX,
18 an international standard for UNIX-like systems.
19 As of this writing, the current edition of POSIX is:
20
21 Standard for Information technology
22 -- Portable Operating System Interface (POSIX (R))
23 -- System Interfaces
24 IEEE Std 1003.1, 2004 Edition
25 <http://www.opengroup.org/online-pubs?DOC=7999959899>
26 <http://www.opengroup.org/pubs/catalog/t041.htm>
27
28 POSIX has the following properties and limitations.
29
30 * In POSIX, time display in a process is controlled by the
31 environment variable TZ. Unfortunately, the POSIX TZ string takes
32 a form that is hard to describe and is error-prone in practice.
33 Also, POSIX TZ strings can't deal with other (for example, Israeli)
34 daylight saving time rules, or situations where more than two
35 time zone abbreviations are used in an area.
36
37 The POSIX TZ string takes the following form:
38
39 stdoffset[dst[offset],date[/time],date[/time]]
40
41 where:
42
43 std and dst
44 are 3 or more characters specifying the standard
45 and daylight saving time (DST) zone names.
46 Starting with POSIX.1-2001, std and dst may also be
47 in a quoted form like "<UTC+10>"; this allows
48 "+" and "-" in the names.
49 offset
50 is of the form `[-]hh:[mm[:ss]]' and specifies the
51 offset west of UTC. The default DST offset is one hour
52 ahead of standard time.
53 date[/time],date[/time]
54 specifies the beginning and end of DST. If this is absent,
55 the system supplies its own rules for DST, and these can
56 differ from year to year; typically US DST rules are used.
57 time
58 takes the form `hh:[mm[:ss]]' and defaults to 02:00.
59 date
60 takes one of the following forms:
61 Jn (1<=n<=365)
62 origin-1 day number not counting February 29
63 n (0<=n<=365)
64 origin-0 day number counting February 29 if present
65 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
66 for the dth day of week n of month m of the year,
67 where week 1 is the first week in which day d appears,
68 and `5' stands for the last week in which day d appears
69 (which may be either the 4th or 5th week).
70
71 Here is an example POSIX TZ string, for US Pacific time using rules
72 appropriate from 1987 through 2006:
73
74 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
75
76 This POSIX TZ string is hard to remember, and mishandles time stamps
77 before 1987 and after 2006. With this package you can use this
78 instead:
79
80 TZ='America/Los_Angeles'
81
82 * POSIX does not define the exact meaning of TZ values like "EST5EDT".
83 Typically the current US DST rules are used to interpret such values,
84 but this means that the US DST rules are compiled into each program
85 that does time conversion. This means that when US time conversion
86 rules change (as in the United States in 1987), all programs that
87 do time conversion must be recompiled to ensure proper results.
88
89 * In POSIX, there's no tamper-proof way for a process to learn the
90 system's best idea of local wall clock. (This is important for
91 applications that an administrator wants used only at certain times--
92 without regard to whether the user has fiddled the "TZ" environment
93 variable. While an administrator can "do everything in UTC" to get
94 around the problem, doing so is inconvenient and precludes handling
95 daylight saving time shifts--as might be required to limit phone
96 calls to off-peak hours.)
97
98 * POSIX requires that systems ignore leap seconds.
99
100 These are the extensions that have been made to the POSIX functions:
101
102 * The "TZ" environment variable is used in generating the name of a file
103 from which time zone information is read (or is interpreted a la
104 POSIX); "TZ" is no longer constrained to be a three-letter time zone
105 name followed by a number of hours and an optional three-letter
106 daylight time zone name. The daylight saving time rules to be used
107 for a particular time zone are encoded in the time zone file;
108 the format of the file allows U.S., Australian, and other rules to be
109 encoded, and allows for situations where more than two time zone
110 abbreviations are used.
111
112 It was recognized that allowing the "TZ" environment variable to
113 take on values such as "America/New_York" might cause "old" programs
114 (that expect "TZ" to have a certain form) to operate incorrectly;
115 consideration was given to using some other environment variable
116 (for example, "TIMEZONE") to hold the string used to generate the
117 time zone information file name. In the end, however, it was decided
118 to continue using "TZ": it is widely used for time zone purposes;
119 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
120 and systems where "new" forms of "TZ" might cause problems can simply
121 use TZ values such as "EST5EDT" which can be used both by
122 "new" programs (a la POSIX) and "old" programs (as zone names and
123 offsets).
124
125 * To handle places where more than two time zone abbreviations are used,
126 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
127 (where "tmp" is the value the function returns) to the time zone
128 abbreviation to be used. This differs from POSIX, where the elements
129 of tzname are only changed as a result of calls to tzset.
130
131 * Since the "TZ" environment variable can now be used to control time
132 conversion, the "daylight" and "timezone" variables are no longer
133 needed. (These variables are defined and set by "tzset"; however, their
134 values will not be used by "localtime.")
135
136 * The "localtime" function has been set up to deliver correct results
137 for near-minimum or near-maximum time_t values. (A comment in the
138 source code tells how to get compatibly wrong results).
139
140 * A function "tzsetwall" has been added to arrange for the system's
141 best approximation to local wall clock time to be delivered by
142 subsequent calls to "localtime." Source code for portable
143 applications that "must" run on local wall clock time should call
144 "tzsetwall();" if such code is moved to "old" systems that don't
145 provide tzsetwall, you won't be able to generate an executable program.
146 (These time zone functions also arrange for local wall clock time to be
147 used if tzset is called--directly or indirectly--and there's no "TZ"
148 environment variable; portable applications should not, however, rely
149 on this behavior since it's not the way SVR2 systems behave.)
150
151 * These functions can account for leap seconds, thanks to Bradley White.
152
153 Points of interest to folks with other systems:
154
155 * This package is already part of many POSIX-compliant hosts,
156 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
157 On such hosts, the primary use of this package
158 is to update obsolete time zone rule tables.
159 To do this, you may need to compile the time zone compiler
160 `zic' supplied with this package instead of using the system `zic',
161 since the format of zic's input changed slightly in late 1994,
162 and many vendors still do not support the new input format.
163
164 * The UNIX Version 7 "timezone" function is not present in this package;
165 it's impossible to reliably map timezone's arguments (a "minutes west
166 of GMT" value and a "daylight saving time in effect" flag) to a
167 time zone abbreviation, and we refuse to guess.
168 Programs that in the past used the timezone function may now examine
169 tzname[localtime(&clock)->tm_isdst] to learn the correct time
170 zone abbreviation to use. Alternatively, use
171 localtime(&clock)->tm_zone if this has been enabled.
172
173 * The 4.2BSD gettimeofday function is not used in this package.
174 This formerly let users obtain the current UTC offset and DST flag,
175 but this functionality was removed in later versions of BSD.
176
177 * In SVR2, time conversion fails for near-minimum or near-maximum
178 time_t values when doing conversions for places that don't use UTC.
179 This package takes care to do these conversions correctly.
180
181 The functions that are conditionally compiled if STD_INSPIRED is defined
182 should, at this point, be looked on primarily as food for thought. They are
183 not in any sense "standard compatible"--some are not, in fact, specified in
184 *any* standard. They do, however, represent responses of various authors to
185 standardization proposals.
186
187 Other time conversion proposals, in particular the one developed by folks at
188 Hewlett Packard, offer a wider selection of functions that provide capabilities
189 beyond those provided here. The absence of such functions from this package
190 is not meant to discourage the development, standardization, or use of such
191 functions. Rather, their absence reflects the decision to make this package
192 contain valid extensions to POSIX, to ensure its broad acceptability. If
193 more powerful time conversion functions can be standardized, so much the
194 better.
195
196
197 ----- Scope of the tz database -----
198
199 The tz database attempts to record the history and predicted future of
200 all computer-based clocks that track civil time. To represent this
201 data, the world is partitioned into regions whose clocks all agree
202 about time stamps that occur after the somewhat-arbitrary cutoff point
203 of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region,
204 the database records all known clock transitions, and labels the region
205 with a notable location.
206
207 Clock transitions before 1970 are recorded for each such location,
208 because most POSIX-compatible systems support negative time stamps and
209 could misbehave if data were omitted for pre-1970 transitions.
210 However, the database is not designed for and does not suffice for
211 applications requiring accurate handling of all past times everywhere,
212 as it would take far too much effort and guesswork to record all
213 details of pre-1970 civil timekeeping.
214
215 As noted in the README file, the tz database is not authoritative
216 (particularly not for pre-1970 time stamps), and it surely has errors.
217 Corrections are welcome and encouraged. Users requiring authoritative
218 data should consult national standards bodies and the references cited
219 in the database's comments.
220
221
222 ----- Names of time zone rule files -----
223
224 The time zone rule file naming conventions attempt to strike a balance
225 among the following goals:
226
227 * Uniquely identify every national region where clocks have all
228 agreed since 1970. This is essential for the intended use: static
229 clocks keeping local civil time.
230
231 * Indicate to humans as to where that region is. This simplifes use.
232
233 * Be robust in the presence of political changes. This reduces the
234 number of updates and backward-compatibility hacks. For example,
235 names of countries are ordinarily not used, to avoid
236 incompatibilities when countries change their name
237 (e.g. Zaire->Congo) or when locations change countries
238 (e.g. Hong Kong from UK colony to China).
239
240 * Be portable to a wide variety of implementations.
241 This promotes use of the technology.
242
243 * Use a consistent naming convention over the entire world.
244 This simplifies both use and maintenance.
245
246 This naming convention is not intended for use by inexperienced users
247 to select TZ values by themselves (though they can of course examine
248 and reuse existing settings). Distributors should provide
249 documentation and/or a simple selection interface that explains the
250 names; see the 'tzselect' program supplied with this distribution for
251 one example.
252
253 Names normally have the form AREA/LOCATION, where AREA is the name
254 of a continent or ocean, and LOCATION is the name of a specific
255 location within that region. North and South America share the same
256 area, `America'. Typical names are `Africa/Cairo', `America/New_York',
257 and `Pacific/Honolulu'.
258
259 Here are the general rules used for choosing location names,
260 in decreasing order of importance:
261
262 Use only valid POSIX file name components (i.e., the parts of
263 names other than `/'). Within a file name component,
264 use only ASCII letters, `.', `-' and `_'. Do not use
265 digits, as that might create an ambiguity with POSIX
266 TZ strings. A file name component must not exceed 14
267 characters or start with `-'. E.g., prefer `Brunei'
268 to `Bandar_Seri_Begawan'.
269 Include at least one location per time zone rule set per country.
270 One such location is enough. Use ISO 3166 (see the file
271 iso3166.tab) to help decide whether something is a country.
272 However, uninhabited ISO 3166 regions like Bouvet Island
273 do not need locations, since local time is not defined there.
274 If all the clocks in a country's region have agreed since 1970,
275 don't bother to include more than one location
276 even if subregions' clocks disagreed before 1970.
277 Otherwise these tables would become annoyingly large.
278 If a name is ambiguous, use a less ambiguous alternative;
279 e.g. many cities are named San Jose and Georgetown, so
280 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'.
281 Keep locations compact. Use cities or small islands, not countries
282 or regions, so that any future time zone changes do not split
283 locations into different time zones. E.g. prefer `Paris'
284 to `France', since France has had multiple time zones.
285 Use mainstream English spelling, e.g. prefer `Rome' to `Roma', and
286 prefer `Athens' to the true name (which uses Greek letters).
287 The POSIX file name restrictions encourage this rule.
288 Use the most populous among locations in a country's time zone,
289 e.g. prefer `Shanghai' to `Beijing'. Among locations with
290 similar populations, pick the best-known location,
291 e.g. prefer `Rome' to `Milan'.
292 Use the singular form, e.g. prefer `Canary' to `Canaries'.
293 Omit common suffixes like `_Islands' and `_City', unless that
294 would lead to ambiguity. E.g. prefer `Cayman' to
295 `Cayman_Islands' and `Guatemala' to `Guatemala_City',
296 but prefer `Mexico_City' to `Mexico' because the country
297 of Mexico has several time zones.
298 Use `_' to represent a space.
299 Omit `.' from abbreviations in names, e.g. prefer `St_Helena'
300 to `St._Helena'.
301 Do not change established names if they only marginally
302 violate the above rules. For example, don't change
303 the existing name `Rome' to `Milan' merely because
304 Milan's population has grown to be somewhat greater
305 than Rome's.
306 If a name is changed, put its old spelling in the `backward' file.
307
308 The file `zone.tab' lists the geographical locations used to name
309 time zone rule files. It is intended to be an exhaustive list
310 of canonical names for geographic regions.
311
312 Older versions of this package used a different naming scheme,
313 and these older names are still supported.
314 See the file `backward' for most of these older names
315 (e.g. `US/Eastern' instead of `America/New_York').
316 The other old-fashioned names still supported are
317 +`WET', `CET', `MET', and `EET' (see the file `europe').
318
319
320 ----- Time zone abbreviations -----
321
322 When this package is installed, it generates time zone abbreviations
323 like `EST' to be compatible with human tradition and POSIX.
324 Here are the general rules used for choosing time zone abbreviations,
325 in decreasing order of importance:
326
327 Use abbreviations that consist of three or more ASCII letters.
328 Previous editions of this database also used characters like
329 ' ' and '?', but these characters have a special meaning to
330 the shell and cause commands like
331 set `date`
332 to have unexpected effects.
333 Previous editions of this rule required upper-case letters,
334 but the Congressman who introduced Chamorro Standard Time
335 preferred "ChST", so the rule has been relaxed.
336
337 This rule guarantees that all abbreviations could have
338 been specified by a POSIX TZ string. POSIX
339 requires at least three characters for an
340 abbreviation. POSIX through 2000 says that an abbreviation
341 cannot start with ':', and cannot contain ',', '-',
342 '+', NUL, or a digit. POSIX from 2001 on changes this
343 rule to say that an abbreviation can contain only '-', '+',
344 and alphanumeric characters from the portable character set
345 in the current locale. To be portable to both sets of
346 rules, an abbreviation must therefore use only ASCII
347 letters.
348
349 Use abbreviations that are in common use among English-speakers,
350 e.g. `EST' for Eastern Standard Time in North America.
351 We assume that applications translate them to other languages
352 as part of the normal localization process; for example,
353 a French application might translate `EST' to `HNE'.
354
355 For zones whose times are taken from a city's longitude, use the
356 traditional xMT notation, e.g. `PMT' for Paris Mean Time.
357 The only name like this in current use is `GMT'.
358
359 If there is no common English abbreviation, abbreviate the English
360 translation of the usual phrase used by native speakers.
361 If this is not available or is a phrase mentioning the country
362 (e.g. ``Cape Verde Time''), then:
363
364 When a country has a single or principal time zone region,
365 append `T' to the country's ISO code, e.g. `CVT' for
366 Cape Verde Time. For summer time append `ST';
367 for double summer time append `DST'; etc.
368 When a country has multiple time zones, take the first three
369 letters of an English place name identifying each zone
370 and then append `T', `ST', etc. as before;
371 e.g. `VLAST' for VLAdivostok Summer Time.
372
373 Use UTC (with time zone abbreviation "zzz") for locations while
374 uninhabited. The "zzz" mnemonic is that these locations are,
375 in some sense, asleep.
376
377 Application writers should note that these abbreviations are ambiguous
378 in practice: e.g. `EST' has a different meaning in Australia than
379 it does in the United States. In new applications, it's often better
380 to use numeric UTC offsets like `-0500' instead of time zone
381 abbreviations like `EST'; this avoids the ambiguity.
382
383
384 ----- Calendrical issues -----
385
386 Calendrical issues are a bit out of scope for a time zone database,
387 but they indicate the sort of problems that we would run into if we
388 extended the time zone database further into the past. An excellent
389 resource in this area is Nachum Dershowitz and Edward M. Reingold,
390 <a href="http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/">
391 Calendrical Calculations: Third Edition
392 </a>, Cambridge University Press (2008). Other information and
393 sources are given below. They sometimes disagree.
394
395
396 France
397
398 Gregorian calendar adopted 1582-12-20.
399 French Revolutionary calendar used 1793-11-24 through 1805-12-31,
400 and (in Paris only) 1871-05-06 through 1871-05-23.
401
402
403 Russia
404
405 From Chris Carrier (1996-12-02):
406 On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar''
407 with 30-day months plus 5 holidays, with a 5-day week.
408 On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
409 Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
410 reverted to the 7-day week. With the 6-day week the usual days
411 off were the 6th, 12th, 18th, 24th and 30th of the month.
412 (Source: Evitiar Zerubavel, _The Seven Day Circle_)
413
414
415 Mark Brader reported a similar story in "The Book of Calendars", edited
416 by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But:
417
418 From: Petteri Sulonen (via Usenet)
419 Date: 14 Jan 1999 00:00:00 GMT
420 ...
421
422 If your source is correct, how come documents between 1929 -- 1940 were
423 still dated using the conventional, Gregorian calendar?
424
425 I can post a scan of a document dated December 1, 1934, signed by
426 Yenukidze, the secretary, on behalf of Kalinin, the President of the
427 Executive Committee of the Supreme Soviet, if you like.
428
429
430
431 Sweden (and Finland)
432
433 From: Mark Brader
434 <a href="news:1996Jul6.012937.29190 (a] sq.com">
435 Subject: Re: Gregorian reform -- a part of locale?
436 </a>
437 Date: 1996-07-06
438
439 In 1700, Denmark made the transition from Julian to Gregorian. Sweden
440 decided to *start* a transition in 1700 as well, but rather than have one of
441 those unsightly calendar gaps :-), they simply decreed that the next leap
442 year after 1696 would be in 1744 -- putting the whole country on a calendar
443 different from both Julian and Gregorian for a period of 40 years.
444
445 However, in 1704 something went wrong and the plan was not carried through;
446 they did, after all, have a leap year that year. And one in 1708. In 1712
447 they gave it up and went back to Julian, putting 30 days in February that
448 year!...
449
450 Then in 1753, Sweden made the transition to Gregorian in the usual manner,
451 getting there only 13 years behind the original schedule.
452
453 (A previous posting of this story was challenged, and Swedish readers
454 produced the following references to support it: "Tiderakning och historia"
455 by Natanael Beckman (1924) and "Tid, en bok om tiderakning och
456 kalendervasen" by Lars-Olof Lode'n (no date was given).)
457
458
459 Grotefend's data
460
461 From: "Michael Palmer" [with one obvious typo fixed]
462 Subject: Re: Gregorian Calendar (was Re: Another FHC related question
463 Newsgroups: soc.genealogy.german
464 Date: Tue, 9 Feb 1999 02:32:48 -800
465 ...
466
467 The following is a(n incomplete) listing, arranged chronologically, of
468 European states, with the date they converted from the Julian to the
469 Gregorian calendar:
470
471 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
472 Catholics and Danzig only)
473 09/20 Dec 1582 - France, Lorraine
474
475 21 Dec 1582/
476 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
477 10/21 Feb 1583 - bishopric of Liege (L"uttich)
478 13/24 Feb 1583 - bishopric of Augsburg
479 04/15 Oct 1583 - electorate of Trier
480 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
481 Salzburg, Brixen
482 13/24 Oct 1583 - Austrian Oberelsass and Breisgau
483 20/31 Oct 1583 - bishopric of Basel
484 02/13 Nov 1583 - duchy of J"ulich-Berg
485 02/13 Nov 1583 - electorate and city of K"oln
486 04/15 Nov 1583 - bishopric of W"urzburg
487 11/22 Nov 1583 - electorate of Mainz
488 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
489 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve
490 14/25 Dec 1583 - Steiermark
491
492 06/17 Jan 1584 - Austria and Bohemia
493 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn
494 12/23 Jan 1584 - Silesia and the Lausitz
495 22 Jan/
496 02 Feb 1584 - Hungary (legally on 21 Oct 1587)
497 Jun 1584 - Unterwalden
498 01/12 Jul 1584 - duchy of Westfalen
499
500 16/27 Jun 1585 - bishopric of Paderborn
501
502 14/25 Dec 1590 - Transylvania
503
504 22 Aug/
505 02 Sep 1612 - duchy of Prussia
506
507 13/24 Dec 1614 - Pfalz-Neuburg
508
509 1617 - duchy of Kurland (reverted to the Julian calendar in
510 1796)
511
512 1624 - bishopric of Osnabr"uck
513
514 1630 - bishopric of Minden
515
516 15/26 Mar 1631 - bishopric of Hildesheim
517
518 1655 - Kanton Wallis
519
520 05/16 Feb 1682 - city of Strassburg
521
522 18 Feb/
523 01 Mar 1700 - Protestant Germany (including Swedish possessions in
524 Germany), Denmark, Norway
525 30 Jun/
526 12 Jul 1700 - Gelderland, Zutphen
527 10 Nov/
528 12 Dec 1700 - Utrecht, Overijssel
529
530 31 Dec 1700/
531 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva,
532 Turgau, and Schaffhausen
533
534 1724 - Glarus, Appenzell, and the city of St. Gallen
535
536 01 Jan 1750 - Pisa and Florence
537
538 02/14 Sep 1752 - Great Britain
539
540 17 Feb/
541 01 Mar 1753 - Sweden
542
543 1760-1812 - Graub"unden
544
545 The Russian empire (including Finland and the Baltic states) did not
546 convert to the Gregorian calendar until the Soviet revolution of 1917.
547
548 Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
549 Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
550 (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
551
552
553 ----- Time and time zones on Mars -----
554
555 Some people have adjusted their work schedules to fit Mars time.
556 Dozens of special Mars watches were built for Jet Propulsion
557 Laboratory workers who kept Mars time during the Mars Exploration
558 Rovers mission (2004). These timepieces look like normal Seikos and
559 Citizens but use Mars seconds rather than terrestrial seconds.
560
561 A Mars solar day is called a "sol" and has a mean period equal to
562 about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
563 divided into a conventional 24-hour clock, so each Mars second equals
564 about 1.02749125 terrestrial seconds.
565
566 The prime meridian of Mars goes through the center of the crater
567 Airy-0, named in honor of the British astronomer who built the
568 Greenwich telescope that defines Earth's prime meridian. Mean solar
569 time on the Mars prime meridian is called Mars Coordinated Time (MTC).
570
571 Each landed mission on Mars has adopted a different reference for
572 solar time keeping, so there is no real standard for Mars time zones.
573 For example, the Mars Exploration Rover project (2004) defined two
574 time zones "Local Solar Time A" and "Local Solar Time B" for its two
575 missions, each zone designed so that its time equals local true solar
576 time at approximately the middle of the nominal mission. Such a "time
577 zone" is not particularly suited for any application other than the
578 mission itself.
579
580 Many calendars have been proposed for Mars, but none have achieved
581 wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
582 sequential count of Mars solar days elapsed since about 1873-12-29
583 12:00 GMT.
584
585 The tz database does not currently support Mars time, but it is
586 documented here in the hopes that support will be added eventually.
587
588 Sources:
589
590 Michael Allison and Robert Schmunk,
591 "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
592 <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-07-30).
593
594 Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
595 (2004-01-14), pp A1, A20-A21.
596