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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