Sync zoneinfo database with tz{code,data}2009t from elsie.
[dragonfly.git] / share / zoneinfo / Theory
CommitLineData
cf207d0a 1@(#)Theory 8.4
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2This file is in the public domain, so clarified as of
32009-05-17 by Arthur David Olson.
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4
5----- Outline -----
6
7 Time and date functions
8 Names of time zone regions
9 Time zone abbreviations
10 Calendrical issues
ba5ba9da 11 Time and time zones on Mars
984263bc 12
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13----- Time and date functions -----
14
9ffe4b14 15These time and date functions are upwards compatible with POSIX,
ba5ba9da 16an international standard for UNIX-like systems.
9ffe4b14 17As of this writing, the current edition of POSIX is:
984263bc 18
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19 Standard for Information technology
20 -- Portable Operating System Interface (POSIX (R))
21 -- System Interfaces
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>
984263bc 25
9ffe4b14 26POSIX has the following properties and limitations.
984263bc 27
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28* In POSIX, time display in a process is controlled by the
29 environment variable TZ. Unfortunately, the POSIX TZ string takes
984263bc 30 a form that is hard to describe and is error-prone in practice.
9ffe4b14 31 Also, POSIX TZ strings can't deal with other (for example, Israeli)
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32 daylight saving time rules, or situations where more than two
33 time zone abbreviations are used in an area.
34
9ffe4b14 35 The POSIX TZ string takes the following form:
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36
37 stdoffset[dst[offset],date[/time],date[/time]]
38
39 where:
40
41 std and dst
42 are 3 or more characters specifying the standard
43 and daylight saving time (DST) zone names.
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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.
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47 offset
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.
55 time
56 takes the form `hh:[mm[:ss]]' and defaults to 02:00.
57 date
58 takes one of the following forms:
59 Jn (1<=n<=365)
60 origin-1 day number not counting February 29
61 n (0<=n<=365)
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).
8393cd87 68
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69 Here is an example POSIX TZ string, for US Pacific time using rules
70 appropriate from 1987 through 2006:
984263bc 71
8393cd87 72 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
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73
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
76 instead:
77
78 TZ='America/Los_Angeles'
79
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,
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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.
86
9ffe4b14 87* In POSIX, there's no tamper-proof way for a process to learn the
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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.)
95
9ffe4b14 96* POSIX requires that systems ignore leap seconds.
984263bc 97
9ffe4b14 98These are the extensions that have been made to the POSIX functions:
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99
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.
109
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
121 offsets).
122
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
9ffe4b14 126 abbreviation to be used. This differs from POSIX, where the elements
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127 of tzname are only changed as a result of calls to tzset.
128
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.")
133
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).
137
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.)
148
9ffe4b14 149* These functions can account for leap seconds, thanks to Bradley White.
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150
151Points of interest to folks with other systems:
152
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.
161
ba5ba9da 162* The UNIX Version 7 "timezone" function is not present in this package;
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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.
170
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.
174
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.
178
179The functions that are conditionally compiled if STD_INSPIRED is defined
180should, at this point, be looked on primarily as food for thought. They are
181not in any sense "standard compatible"--some are not, in fact, specified in
182*any* standard. They do, however, represent responses of various authors to
183standardization proposals.
184
185Other time conversion proposals, in particular the one developed by folks at
186Hewlett Packard, offer a wider selection of functions that provide capabilities
187beyond those provided here. The absence of such functions from this package
188is not meant to discourage the development, standardization, or use of such
189functions. Rather, their absence reflects the decision to make this package
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190contain valid extensions to POSIX, to ensure its broad acceptability. If
191more powerful time conversion functions can be standardized, so much the
192better.
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193
194
195----- Names of time zone rule files -----
196
197The time zone rule file naming conventions attempt to strike a balance
198among the following goals:
199
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.
203
204 * Indicate to humans as to where that region is. This simplifes use.
205
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).
212
213 * Be portable to a wide variety of implementations.
214 This promotes use of the technology.
215
216 * Use a consistent naming convention over the entire world.
217 This simplifies both use and maintenance.
218
219This naming convention is not intended for use by inexperienced users
220to select TZ values by themselves (though they can of course examine
221and reuse existing settings). Distributors should provide
222documentation and/or a simple selection interface that explains the
223names; see the 'tzselect' program supplied with this distribution for
224one example.
225
226Names normally have the form AREA/LOCATION, where AREA is the name
227of a continent or ocean, and LOCATION is the name of a specific
228location within that region. North and South America share the same
229area, `America'. Typical names are `Africa/Cairo', `America/New_York',
230and `Pacific/Honolulu'.
231
232Here are the general rules used for choosing location names,
233in decreasing order of importance:
234
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.
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245 However, uninhabited ISO 3166 regions like Bouvet Island
246 do not need locations, since local time is not defined there.
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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'
273 to `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
278 than Rome's.
279 If a name is changed, put its old spelling in the `backward' file.
280
281The file `zone.tab' lists the geographical locations used to name
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282time zone rule files. It is intended to be an exhaustive list
283of canonical names for geographic regions.
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284
285Older versions of this package used a different naming scheme,
286and these older names are still supported.
287See the file `backward' for most of these older names
288(e.g. `US/Eastern' instead of `America/New_York').
289The other old-fashioned names still supported are
290`WET', `CET', `MET', `EET' (see the file `europe'),
291and `Factory' (see the file `factory').
292
293
294----- Time zone abbreviations -----
295
296When this package is installed, it generates time zone abbreviations
9ffe4b14 297like `EST' to be compatible with human tradition and POSIX.
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298Here are the general rules used for choosing time zone abbreviations,
299in decreasing order of importance:
300
301 Use abbreviations that consist of three or more ASCII letters.
302 Previous editions of this database also used characters like
303 ' ' and '?', but these characters have a special meaning to
304 the shell and cause commands like
305 set `date`
306 to have unexpected effects.
307 Previous editions of this rule required upper-case letters,
308 but the Congressman who introduced Chamorro Standard Time
309 preferred "ChST", so the rule has been relaxed.
310
311 This rule guarantees that all abbreviations could have
9ffe4b14 312 been specified by a POSIX TZ string. POSIX
984263bc 313 requires at least three characters for an
9ffe4b14 314 abbreviation. POSIX through 2000 says that an abbreviation
984263bc 315 cannot start with ':', and cannot contain ',', '-',
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316 '+', NUL, or a digit. POSIX from 2001 on changes this
317 rule to say that an abbreviation can contain only '-', '+',
318 and alphanumeric characters from the portable character set
319 in the current locale. To be portable to both sets of
984263bc 320 rules, an abbreviation must therefore use only ASCII
9ffe4b14 321 letters.
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322
323 Use abbreviations that are in common use among English-speakers,
324 e.g. `EST' for Eastern Standard Time in North America.
325 We assume that applications translate them to other languages
326 as part of the normal localization process; for example,
327 a French application might translate `EST' to `HNE'.
328
329 For zones whose times are taken from a city's longitude, use the
330 traditional xMT notation, e.g. `PMT' for Paris Mean Time.
331 The only name like this in current use is `GMT'.
332
333 If there is no common English abbreviation, abbreviate the English
334 translation of the usual phrase used by native speakers.
335 If this is not available or is a phrase mentioning the country
336 (e.g. ``Cape Verde Time''), then:
337
338 When a country has a single or principal time zone region,
339 append `T' to the country's ISO code, e.g. `CVT' for
340 Cape Verde Time. For summer time append `ST';
341 for double summer time append `DST'; etc.
342 When a country has multiple time zones, take the first three
343 letters of an English place name identifying each zone
344 and then append `T', `ST', etc. as before;
345 e.g. `VLAST' for VLAdivostok Summer Time.
346
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347 Use UTC (with time zone abbreviation "zzz") for locations while
348 uninhabited. The "zzz" mnemonic is that these locations are,
349 in some sense, asleep.
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350
351Application writers should note that these abbreviations are ambiguous
352in practice: e.g. `EST' has a different meaning in Australia than
353it does in the United States. In new applications, it's often better
354to use numeric UTC offsets like `-0500' instead of time zone
355abbreviations like `EST'; this avoids the ambiguity.
356
357
358----- Calendrical issues -----
359
360Calendrical issues are a bit out of scope for a time zone database,
361but they indicate the sort of problems that we would run into if we
362extended the time zone database further into the past. An excellent
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363resource in this area is Nachum Dershowitz and Edward M. Reingold,
364<a href="http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/">
365Calendrical Calculations: Third Edition
366</a>, Cambridge University Press (2008). Other information and
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367sources are given below. They sometimes disagree.
368
369
370France
371
372Gregorian calendar adopted 1582-12-20.
373French Revolutionary calendar used 1793-11-24 through 1805-12-31,
374and (in Paris only) 1871-05-06 through 1871-05-23.
375
376
377Russia
378
9ffe4b14 379From Chris Carrier (1996-12-02):
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380On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar''
381with 30-day months plus 5 holidays, with a 5-day week.
382On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
383Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
384reverted to the 7-day week. With the 6-day week the usual days
385off were the 6th, 12th, 18th, 24th and 30th of the month.
386(Source: Evitiar Zerubavel, _The Seven Day Circle_)
387
388
389Mark Brader reported a similar story in "The Book of Calendars", edited
390by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But:
391
392From: Petteri Sulonen (via Usenet)
393Date: 14 Jan 1999 00:00:00 GMT
9ffe4b14 394...
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395
396If your source is correct, how come documents between 1929 -- 1940 were
397still dated using the conventional, Gregorian calendar?
398
399I can post a scan of a document dated December 1, 1934, signed by
400Yenukidze, the secretary, on behalf of Kalinin, the President of the
401Executive Committee of the Supreme Soviet, if you like.
402
403
404
405Sweden (and Finland)
406
9ffe4b14 407From: Mark Brader
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408<a href="news:1996Jul6.012937.29190@sq.com">
409Subject: Re: Gregorian reform -- a part of locale?
410</a>
411Date: 1996-07-06
412
413In 1700, Denmark made the transition from Julian to Gregorian. Sweden
414decided to *start* a transition in 1700 as well, but rather than have one of
415those unsightly calendar gaps :-), they simply decreed that the next leap
416year after 1696 would be in 1744 -- putting the whole country on a calendar
417different from both Julian and Gregorian for a period of 40 years.
418
419However, in 1704 something went wrong and the plan was not carried through;
420they did, after all, have a leap year that year. And one in 1708. In 1712
421they gave it up and went back to Julian, putting 30 days in February that
422year!...
423
424Then in 1753, Sweden made the transition to Gregorian in the usual manner,
425getting there only 13 years behind the original schedule.
426
427(A previous posting of this story was challenged, and Swedish readers
428produced the following references to support it: "Tiderakning och historia"
429by Natanael Beckman (1924) and "Tid, en bok om tiderakning och
430kalendervasen" by Lars-Olof Lode'n (no date was given).)
431
432
433Grotefend's data
434
9ffe4b14 435From: "Michael Palmer" [with one obvious typo fixed]
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436Subject: Re: Gregorian Calendar (was Re: Another FHC related question
437Newsgroups: soc.genealogy.german
438Date: Tue, 9 Feb 1999 02:32:48 -800
9ffe4b14 439...
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440
441The following is a(n incomplete) listing, arranged chronologically, of
442European states, with the date they converted from the Julian to the
443Gregorian calendar:
444
44504/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
446 Catholics and Danzig only)
44709/20 Dec 1582 - France, Lorraine
448
44921 Dec 1582/
450 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
45110/21 Feb 1583 - bishopric of Liege (L"uttich)
45213/24 Feb 1583 - bishopric of Augsburg
45304/15 Oct 1583 - electorate of Trier
45405/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
455 Salzburg, Brixen
45613/24 Oct 1583 - Austrian Oberelsass and Breisgau
45720/31 Oct 1583 - bishopric of Basel
45802/13 Nov 1583 - duchy of J"ulich-Berg
45902/13 Nov 1583 - electorate and city of K"oln
46004/15 Nov 1583 - bishopric of W"urzburg
46111/22 Nov 1583 - electorate of Mainz
46216/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
46317/28 Nov 1583 - bishopric of M"unster and duchy of Cleve
46414/25 Dec 1583 - Steiermark
465
46606/17 Jan 1584 - Austria and Bohemia
46711/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn
46812/23 Jan 1584 - Silesia and the Lausitz
46922 Jan/
470 02 Feb 1584 - Hungary (legally on 21 Oct 1587)
471 Jun 1584 - Unterwalden
47201/12 Jul 1584 - duchy of Westfalen
473
47416/27 Jun 1585 - bishopric of Paderborn
475
47614/25 Dec 1590 - Transylvania
477
47822 Aug/
479 02 Sep 1612 - duchy of Prussia
480
48113/24 Dec 1614 - Pfalz-Neuburg
482
483 1617 - duchy of Kurland (reverted to the Julian calendar in
484 1796)
485
486 1624 - bishopric of Osnabr"uck
487
488 1630 - bishopric of Minden
489
49015/26 Mar 1631 - bishopric of Hildesheim
491
492 1655 - Kanton Wallis
493
49405/16 Feb 1682 - city of Strassburg
495
49618 Feb/
497 01 Mar 1700 - Protestant Germany (including Swedish possessions in
498 Germany), Denmark, Norway
49930 Jun/
500 12 Jul 1700 - Gelderland, Zutphen
50110 Nov/
502 12 Dec 1700 - Utrecht, Overijssel
503
50431 Dec 1700/
505 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva,
506 Turgau, and Schaffhausen
507
508 1724 - Glarus, Appenzell, and the city of St. Gallen
509
51001 Jan 1750 - Pisa and Florence
511
51202/14 Sep 1752 - Great Britain
513
51417 Feb/
515 01 Mar 1753 - Sweden
516
5171760-1812 - Graub"unden
518
519The Russian empire (including Finland and the Baltic states) did not
520convert to the Gregorian calendar until the Soviet revolution of 1917.
521
522Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
523Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
524(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
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525
526
527----- Time and time zones on Mars -----
528
529Some people have adjusted their work schedules to fit Mars time.
530Dozens of special Mars watches were built for Jet Propulsion
531Laboratory workers who kept Mars time during the Mars Exploration
532Rovers mission (2004). These timepieces look like normal Seikos and
533Citizens but use Mars seconds rather than terrestrial seconds.
534
535A Mars solar day is called a "sol" and has a mean period equal to
536about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
537divided into a conventional 24-hour clock, so each Mars second equals
538about 1.02749125 terrestrial seconds.
539
540The prime meridian of Mars goes through the center of the crater
541Airy-0, named in honor of the British astronomer who built the
542Greenwich telescope that defines Earth's prime meridian. Mean solar
543time on the Mars prime meridian is called Mars Coordinated Time (MTC).
544
545Each landed mission on Mars has adopted a different reference for
546solar time keeping, so there is no real standard for Mars time zones.
547For example, the Mars Exploration Rover project (2004) defined two
548time zones "Local Solar Time A" and "Local Solar Time B" for its two
549missions, each zone designed so that its time equals local true solar
550time at approximately the middle of the nominal mission. Such a "time
551zone" is not particularly suited for any application other than the
552mission itself.
553
554Many calendars have been proposed for Mars, but none have achieved
555wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
556sequential count of Mars solar days elapsed since about 1873-12-29
55712:00 GMT.
558
559The tz database does not currently support Mars time, but it is
560documented here in the hopes that support will be added eventually.
561
562Sources:
563
564Michael Allison and Robert Schmunk,
565"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
9ffe4b14 566<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-07-30).
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567
568Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
569(2004-01-14), pp A1, A20-A21.