Theory revision 1.15
1This file is in the public domain, so clarified as of 22009-05-17 by Arthur David Olson. 3 4----- Outline ----- 5 6 Time and date functions 7 Scope of the tz database 8 Names of time zone rule files 9 Time zone abbreviations 10 Calendrical issues 11 Time and time zones on Mars 12 13----- Time and date functions ----- 14 15These time and date functions are upwards compatible with those of POSIX, 16an international standard for UNIX-like systems. 17As of this writing, the current edition of POSIX is: 18 19 The Open Group Base Specifications Issue 7 20 IEEE Std 1003.1, 2013 Edition 21 <http://pubs.opengroup.org/onlinepubs/9699919799/> 22 23POSIX has the following properties and limitations. 24 25* In POSIX, time display in a process is controlled by the 26 environment variable TZ. Unfortunately, the POSIX TZ string takes 27 a form that is hard to describe and is error-prone in practice. 28 Also, POSIX TZ strings can't deal with other (for example, Israeli) 29 daylight saving time rules, or situations where more than two 30 time zone abbreviations are used in an area. 31 32 The POSIX TZ string takes the following form: 33 34 stdoffset[dst[offset][,date[/time],date[/time]]] 35 36 where: 37 38 std and dst 39 are 3 or more characters specifying the standard 40 and daylight saving time (DST) zone names. 41 Starting with POSIX.1-2001, std and dst may also be 42 in a quoted form like "<UTC+10>"; this allows 43 "+" and "-" in the names. 44 offset 45 is of the form '[+-]hh:[mm[:ss]]' and specifies the 46 offset west of UT. 'hh' may be a single digit; 0<=hh<=24. 47 The default DST offset is one hour ahead of standard time. 48 date[/time],date[/time] 49 specifies the beginning and end of DST. If this is absent, 50 the system supplies its own rules for DST, and these can 51 differ from year to year; typically US DST rules are used. 52 time 53 takes the form 'hh:[mm[:ss]]' and defaults to 02:00. 54 This is the same format as the offset, except that a 55 leading '+' or '-' is not allowed. 56 date 57 takes one of the following forms: 58 Jn (1<=n<=365) 59 origin-1 day number not counting February 29 60 n (0<=n<=365) 61 origin-0 day number counting February 29 if present 62 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 63 for the dth day of week n of month m of the year, 64 where week 1 is the first week in which day d appears, 65 and '5' stands for the last week in which day d appears 66 (which may be either the 4th or 5th week). 67 Typically, this is the only useful form; 68 the n and Jn forms are rarely used. 69 70 Here is an example POSIX TZ string, for US Pacific time using rules 71 appropriate from 1987 through 2006: 72 73 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00' 74 75 This POSIX TZ string is hard to remember, and mishandles time stamps 76 before 1987 and after 2006. With this package you can use this 77 instead: 78 79 TZ='America/Los_Angeles' 80 81* POSIX does not define the exact meaning of TZ values like "EST5EDT". 82 Typically the current US DST rules are used to interpret such values, 83 but this means that the US DST rules are compiled into each program 84 that does time conversion. This means that when US time conversion 85 rules change (as in the United States in 1987), all programs that 86 do time conversion must be recompiled to ensure proper results. 87 88* In POSIX, there's no tamper-proof way for a process to learn the 89 system's best idea of local wall clock. (This is important for 90 applications that an administrator wants used only at certain times - 91 without regard to whether the user has fiddled the "TZ" environment 92 variable. While an administrator can "do everything in UTC" to get 93 around the problem, doing so is inconvenient and precludes handling 94 daylight saving time shifts - as might be required to limit phone 95 calls to off-peak hours.) 96 97* POSIX requires that systems ignore leap seconds. 98 99* The tz code attempts to support all the time_t implementations 100 allowed by POSIX. The time_t type represents a nonnegative count of 101 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 102 In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit 103 signed time_t values stop working after 2038-01-19 03:14:07 UTC, so 104 new implementations these days typically use a signed 64-bit integer. 105 Unsigned 32-bit integers are used on one or two platforms, 106 and 36-bit integers are also used occasionally. 107 Although earlier POSIX versions allowed time_t to be a 108 floating-point type, this was not supported by any practical 109 systems, and POSIX.1-2013 and the tz code both require time_t 110 to be an integer type. 111 112These are the extensions that have been made to the POSIX functions: 113 114* The "TZ" environment variable is used in generating the name of a file 115 from which time zone information is read (or is interpreted a la 116 POSIX); "TZ" is no longer constrained to be a three-letter time zone 117 name followed by a number of hours and an optional three-letter 118 daylight time zone name. The daylight saving time rules to be used 119 for a particular time zone are encoded in the time zone file; 120 the format of the file allows U.S., Australian, and other rules to be 121 encoded, and allows for situations where more than two time zone 122 abbreviations are used. 123 124 It was recognized that allowing the "TZ" environment variable to 125 take on values such as "America/New_York" might cause "old" programs 126 (that expect "TZ" to have a certain form) to operate incorrectly; 127 consideration was given to using some other environment variable 128 (for example, "TIMEZONE") to hold the string used to generate the 129 time zone information file name. In the end, however, it was decided 130 to continue using "TZ": it is widely used for time zone purposes; 131 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 132 and systems where "new" forms of "TZ" might cause problems can simply 133 use TZ values such as "EST5EDT" which can be used both by 134 "new" programs (a la POSIX) and "old" programs (as zone names and 135 offsets). 136 137* To handle places where more than two time zone abbreviations are used, 138 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 139 (where "tmp" is the value the function returns) to the time zone 140 abbreviation to be used. This differs from POSIX, where the elements 141 of tzname are only changed as a result of calls to tzset. 142 143* Since the "TZ" environment variable can now be used to control time 144 conversion, the "daylight" and "timezone" variables are no longer 145 needed. (These variables are defined and set by "tzset"; however, their 146 values will not be used by "localtime.") 147 148* The "localtime" function has been set up to deliver correct results 149 for near-minimum or near-maximum time_t values. (A comment in the 150 source code tells how to get compatibly wrong results). 151 152* A function "tzsetwall" has been added to arrange for the system's 153 best approximation to local wall clock time to be delivered by 154 subsequent calls to "localtime." Source code for portable 155 applications that "must" run on local wall clock time should call 156 "tzsetwall();" if such code is moved to "old" systems that don't 157 provide tzsetwall, you won't be able to generate an executable program. 158 (These time zone functions also arrange for local wall clock time to be 159 used if tzset is called - directly or indirectly - and there's no "TZ" 160 environment variable; portable applications should not, however, rely 161 on this behavior since it's not the way SVR2 systems behave.) 162 163* Negative time_t values are supported, on systems where time_t is signed. 164 165* These functions can account for leap seconds, thanks to Bradley White. 166 167Points of interest to folks with other systems: 168 169* This package is already part of many POSIX-compliant hosts, 170 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 171 On such hosts, the primary use of this package 172 is to update obsolete time zone rule tables. 173 To do this, you may need to compile the time zone compiler 174 'zic' supplied with this package instead of using the system 'zic', 175 since the format of zic's input changed slightly in late 1994, 176 and many vendors still do not support the new input format. 177 178* The UNIX Version 7 "timezone" function is not present in this package; 179 it's impossible to reliably map timezone's arguments (a "minutes west 180 of GMT" value and a "daylight saving time in effect" flag) to a 181 time zone abbreviation, and we refuse to guess. 182 Programs that in the past used the timezone function may now examine 183 tzname[localtime(&clock)->tm_isdst] to learn the correct time 184 zone abbreviation to use. Alternatively, use 185 localtime(&clock)->tm_zone if this has been enabled. 186 187* The 4.2BSD gettimeofday function is not used in this package. 188 This formerly let users obtain the current UTC offset and DST flag, 189 but this functionality was removed in later versions of BSD. 190 191* In SVR2, time conversion fails for near-minimum or near-maximum 192 time_t values when doing conversions for places that don't use UT. 193 This package takes care to do these conversions correctly. 194 195The functions that are conditionally compiled if STD_INSPIRED is defined 196should, at this point, be looked on primarily as food for thought. They are 197not in any sense "standard compatible" - some are not, in fact, specified in 198*any* standard. They do, however, represent responses of various authors to 199standardization proposals. 200 201Other time conversion proposals, in particular the one developed by folks at 202Hewlett Packard, offer a wider selection of functions that provide capabilities 203beyond those provided here. The absence of such functions from this package 204is not meant to discourage the development, standardization, or use of such 205functions. Rather, their absence reflects the decision to make this package 206contain valid extensions to POSIX, to ensure its broad acceptability. If 207more powerful time conversion functions can be standardized, so much the 208better. 209 210 211----- Scope of the tz database ----- 212 213The tz database attempts to record the history and predicted future of 214all computer-based clocks that track civil time. To represent this 215data, the world is partitioned into regions whose clocks all agree 216about time stamps that occur after the somewhat-arbitrary cutoff point 217of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, 218the database records all known clock transitions, and labels the region 219with a notable location. Although 1970 is a somewhat-arbitrary 220cutoff, there are significant challenges to moving the cutoff earlier 221even by a decade or two, due to the wide variety of local practices 222before computer timekeeping became prevalent. 223 224Clock transitions before 1970 are recorded for each such location, 225because most POSIX-compatible systems support negative time stamps and 226could misbehave if data were omitted for pre-1970 transitions. 227However, the database is not designed for and does not suffice for 228applications requiring accurate handling of all past times everywhere, 229as it would take far too much effort and guesswork to record all 230details of pre-1970 civil timekeeping. 231 232 233----- Accuracy of the tz database ----- 234 235The tz database is not authoritative, and it surely has errors. 236Corrections are welcome and encouraged. Users requiring authoritative 237data should consult national standards bodies and the references cited 238in the database's comments. 239 240Errors in the tz database arise from many sources: 241 242 * The tz database predicts future time stamps, and current predictions 243 will be incorrect after future governments change the rules. 244 For example, if today someone schedules a meeting for 13:00 next 245 October 1, Casablanca time, and tomorrow Morocco changes its 246 daylight saving rules, software can mess up after the rule change 247 if it blithely relies on conversions made before the change. 248 249 * The pre-1970 data in this database cover only a tiny sliver of how 250 clocks actually behaved; the vast majority of the necessary 251 information was lost or never recorded. Thousands more zones would 252 be needed if the tz database's scope were extended to cover even 253 just the known or guessed history of standard time; for example, 254 the current single entry for France would need to split into dozens 255 of entries, perhaps hundreds. 256 257 * Most of the pre-1970 data comes from unreliable sources, often 258 astrology books that lack citations and whose compilers evidently 259 invented entries when the true facts were unknown, without 260 reporting which entries were known and which were invented. 261 These books often contradict each other or give implausible entries, 262 and on the rare occasions when their old data are checked they are 263 typically found to be incorrect. 264 265 * For the UK the tz database relies on years of first-class work done by 266 Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>. 267 Other countries are not done nearly as well. 268 269 * Sometimes, different people in the same city would maintain clocks 270 that differed significantly. Railway time was used by railroad 271 companies (which did not always agree with each other), 272 church-clock time was used for birth certificates, etc. 273 Often this was merely common practice, but sometimes it was set by law. 274 For example, from 1891 to 1911 the UT offset in France was legally 275 0:09:21 outside train stations and 0:04:21 inside. 276 277 * Although a named location in the tz database stands for the 278 containing region, its pre-1970 data entries are often accurate for 279 only a small subset of that region. For example, Europe/London 280 stands for the United Kingdom, but its pre-1847 times are valid 281 only for locations that have London's exact meridian, and its 1847 282 transition to GMT is known to be valid only for the L&NW and the 283 Caledonian railways. 284 285 * The tz database does not record the earliest time for which a 286 zone's data is thereafter valid for every location in the region. 287 For example, Europe/London is valid for all locations in its 288 region after GMT was made the standard time, but the date of 289 standardization (1880-08-02) is not in the tz database, other than 290 in commentary. For many zones the earliest time of validity is 291 unknown. 292 293 * The tz database does not record a region's boundaries, and in many 294 cases the boundaries are not known. For example, the zone 295 America/Kentucky/Louisville represents a region around the city of 296 Louisville, the boundaries of which are unclear. 297 298 * Changes that are modeled as instantaneous transitions in the tz 299 database were often spread out over hours, days, or even decades. 300 301 * Even if the time is specified by law, locations sometimes 302 deliberately flout the law. 303 304 * Early timekeeping practices, even assuming perfect clocks, were 305 often not specified to the accuracy that the tz database requires. 306 307 * Sometimes historical timekeeping was specified more precisely 308 than what the tz database can handle. For example, from 1909 to 309 1937 Netherlands clocks were legally UT+00:19:32.13, but the tz 310 database cannot represent the fractional second. 311 312 * Even when all the timestamp transitions recorded by the tz database 313 are correct, the tz rules that generate them may not faithfully 314 reflect the historical rules. For example, from 1922 until World 315 War II the UK moved clocks forward the day following the third 316 Saturday in April unless that was Easter, in which case it moved 317 clocks forward the previous Sunday. Because the tz database has no 318 way to specify Easter, these exceptional years are entered as 319 separate tz Rule lines, even though the legal rules did not change. 320 321 * The tz database models pre-standard time using the Gregorian 322 calendar and local mean time (LMT), but many people used other 323 calendars and other timescales. For example, the Roman Empire used 324 the Julian calendar, and had 12 varying-length daytime hours with a 325 non-hour-based system at night. 326 327 * Early clocks were less reliable, and the data do not represent this 328 unreliability. 329 330 * As for leap seconds, civil time was not based on atomic time before 331 1972, and we don't know the history of earth's rotation accurately 332 enough to map SI seconds to historical solar time to more than 333 about one-hour accuracy. See: Morrison LV, Stephenson FR. 334 Historical values of the Earth's clock error Delta T and the 335 calculation of eclipses. J Hist Astron. 2004;35:327-36 336 <http://adsabs.harvard.edu/full/2004JHA....35..327M>; 337 Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 338 <http://adsabs.harvard.edu/full/2005JHA....36..339M>. 339 340 * The relationship between POSIX time (that is, UTC but ignoring leap 341 seconds) and UTC is not agreed upon after 1972. Although the POSIX 342 clock officially stops during an inserted leap second, at least one 343 proposed standard has it jumping back a second instead; and in 344 practice POSIX clocks more typically either progress glacially during 345 a leap second, or are slightly slowed while near a leap second. 346 347 * The tz database does not represent how uncertain its information is. 348 Ideally it would contain information about when the data are 349 incomplete or dicey. Partial temporal knowledge is a field of 350 active research, though, and it's not clear how to apply it here. 351 352In short, many, perhaps most, of the tz database's pre-1970 and future 353time stamps are either wrong or misleading. Any attempt to pass the 354tz database off as the definition of time should be unacceptable to 355anybody who cares about the facts. In particular, the tz database's 356LMT offsets should not be considered meaningful, and should not prompt 357creation of zones merely because two locations differ in LMT or 358transitioned to standard time at different dates. 359 360 361----- Names of time zone rule files ----- 362 363The time zone rule file naming conventions attempt to strike a balance 364among the following goals: 365 366 * Uniquely identify every national region where clocks have all 367 agreed since 1970. This is essential for the intended use: static 368 clocks keeping local civil time. 369 370 * Indicate to humans as to where that region is. This simplifies use. 371 372 * Be robust in the presence of political changes. This reduces the 373 number of updates and backward-compatibility hacks. For example, 374 names of countries are ordinarily not used, to avoid 375 incompatibilities when countries change their name 376 (e.g. Zaire->Congo) or when locations change countries 377 (e.g. Hong Kong from UK colony to China). 378 379 * Be portable to a wide variety of implementations. 380 This promotes use of the technology. 381 382 * Use a consistent naming convention over the entire world. 383 This simplifies both use and maintenance. 384 385This naming convention is not intended for use by inexperienced users 386to select TZ values by themselves (though they can of course examine 387and reuse existing settings). Distributors should provide 388documentation and/or a simple selection interface that explains the 389names; see the 'tzselect' program supplied with this distribution for 390one example. 391 392Names normally have the form AREA/LOCATION, where AREA is the name 393of a continent or ocean, and LOCATION is the name of a specific 394location within that region. North and South America share the same 395area, 'America'. Typical names are 'Africa/Cairo', 'America/New_York', 396and 'Pacific/Honolulu'. 397 398Here are the general rules used for choosing location names, 399in decreasing order of importance: 400 401 Use only valid POSIX file name components (i.e., the parts of 402 names other than '/'). Do not use the file name 403 components '.' and '..'. Within a file name component, 404 use only ASCII letters, '.', '-' and '_'. Do not use 405 digits, as that might create an ambiguity with POSIX 406 TZ strings. A file name component must not exceed 14 407 characters or start with '-'. E.g., prefer 'Brunei' 408 to 'Bandar_Seri_Begawan'. Exceptions: see the discussion 409 of legacy names below. 410 A name must not be empty, or contain '//', or start or end with '/'. 411 Do not use names that differ only in case. Although the reference 412 implementation is case-sensitive, some other implementations 413 are not, and they would mishandle names differing only in case. 414 If one name A is an initial prefix of another name AB (ignoring case), 415 then B must not start with '/', as a regular file cannot have 416 the same name as a directory in POSIX. For example, 417 'America/New_York' precludes 'America/New_York/Bronx'. 418 Uninhabited regions like the North Pole and Bouvet Island 419 do not need locations, since local time is not defined there. 420 There should typically be at least one name for each ISO 3166-1 421 officially assigned two-letter code for an inhabited country 422 or territory. 423 If all the clocks in a region have agreed since 1970, 424 don't bother to include more than one location 425 even if subregions' clocks disagreed before 1970. 426 Otherwise these tables would become annoyingly large. 427 If a name is ambiguous, use a less ambiguous alternative; 428 e.g. many cities are named San Jos�� and Georgetown, so 429 prefer 'Costa_Rica' to 'San_Jose' and 'Guyana' to 'Georgetown'. 430 Keep locations compact. Use cities or small islands, not countries 431 or regions, so that any future time zone changes do not split 432 locations into different time zones. E.g. prefer 'Paris' 433 to 'France', since France has had multiple time zones. 434 Use mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and 435 prefer 'Athens' to the Greek '����������' or the Romanized 'Ath��na'. 436 The POSIX file name restrictions encourage this rule. 437 Use the most populous among locations in a zone, 438 e.g. prefer 'Shanghai' to 'Beijing'. Among locations with 439 similar populations, pick the best-known location, 440 e.g. prefer 'Rome' to 'Milan'. 441 Use the singular form, e.g. prefer 'Canary' to 'Canaries'. 442 Omit common suffixes like '_Islands' and '_City', unless that 443 would lead to ambiguity. E.g. prefer 'Cayman' to 444 'Cayman_Islands' and 'Guatemala' to 'Guatemala_City', 445 but prefer 'Mexico_City' to 'Mexico' because the country 446 of Mexico has several time zones. 447 Use '_' to represent a space. 448 Omit '.' from abbreviations in names, e.g. prefer 'St_Helena' 449 to 'St._Helena'. 450 Do not change established names if they only marginally 451 violate the above rules. For example, don't change 452 the existing name 'Rome' to 'Milan' merely because 453 Milan's population has grown to be somewhat greater 454 than Rome's. 455 If a name is changed, put its old spelling in the 'backward' file. 456 This means old spellings will continue to work. 457 458The file 'zone1970.tab' lists geographical locations used to name time 459zone rule files. It is intended to be an exhaustive list of names 460for geographic regions as described above; this is a subset of the 461names in the data. Although a 'zone1970.tab' location's longitude 462corresponds to its LMT offset with one hour for every 15 degrees east 463longitude, this relationship is not exact. 464 465Older versions of this package used a different naming scheme, 466and these older names are still supported. 467See the file 'backward' for most of these older names 468(e.g., 'US/Eastern' instead of 'America/New_York'). 469The other old-fashioned names still supported are 470'WET', 'CET', 'MET', and 'EET' (see the file 'europe'). 471 472Older versions of this package defined legacy names that are 473incompatible with the first rule of location names, but which are 474still supported. These legacy names are mostly defined in the file 475'etcetera'. Also, the file 'backward' defines the legacy names 476'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file 477'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT', 478'MST7MDT', and 'PST8PDT'. 479 480Excluding 'backward' should not affect the other data. If 481'backward' is excluded, excluding 'etcetera' should not affect the 482remaining data. 483 484 485----- Time zone abbreviations ----- 486 487When this package is installed, it generates time zone abbreviations 488like 'EST' to be compatible with human tradition and POSIX. 489Here are the general rules used for choosing time zone abbreviations, 490in decreasing order of importance: 491 492 Use abbreviations that consist of three or more ASCII letters. 493 Previous editions of this database also used characters like 494 ' ' and '?', but these characters have a special meaning to 495 the shell and cause commands like 496 set `date` 497 to have unexpected effects. 498 Previous editions of this rule required upper-case letters, 499 but the Congressman who introduced Chamorro Standard Time 500 preferred "ChST", so the rule has been relaxed. 501 502 This rule guarantees that all abbreviations could have 503 been specified by a POSIX TZ string. POSIX 504 requires at least three characters for an 505 abbreviation. POSIX through 2000 says that an abbreviation 506 cannot start with ':', and cannot contain ',', '-', 507 '+', NUL, or a digit. POSIX from 2001 on changes this 508 rule to say that an abbreviation can contain only '-', '+', 509 and alphanumeric characters from the portable character set 510 in the current locale. To be portable to both sets of 511 rules, an abbreviation must therefore use only ASCII 512 letters. 513 514 Use abbreviations that are in common use among English-speakers, 515 e.g. 'EST' for Eastern Standard Time in North America. 516 We assume that applications translate them to other languages 517 as part of the normal localization process; for example, 518 a French application might translate 'EST' to 'HNE'. 519 520 For zones whose times are taken from a city's longitude, use the 521 traditional xMT notation, e.g. 'PMT' for Paris Mean Time. 522 The only name like this in current use is 'GMT'. 523 524 If there is no common English abbreviation, abbreviate the English 525 translation of the usual phrase used by native speakers. 526 If this is not available or is a phrase mentioning the country 527 (e.g. "Cape Verde Time"), then: 528 529 When a country is identified with a single or principal zone, 530 append 'T' to the country's ISO code, e.g. 'CVT' for 531 Cape Verde Time. For summer time append 'ST'; 532 for double summer time append 'DST'; etc. 533 Otherwise, take the first three letters of an English place 534 name identifying each zone and append 'T', 'ST', etc. 535 as before; e.g. 'VLAST' for VLAdivostok Summer Time. 536 537 Use 'LMT' for local mean time of locations before the introduction 538 of standard time; see "Scope of the tz database". 539 540 Use UT (with time zone abbreviation 'zzz') for locations while 541 uninhabited. The 'zzz' mnemonic is that these locations are, 542 in some sense, asleep. 543 544Application writers should note that these abbreviations are ambiguous 545in practice: e.g. 'CST' has a different meaning in China than 546it does in the United States. In new applications, it's often better 547to use numeric UT offsets like '-0600' instead of time zone 548abbreviations like 'CST'; this avoids the ambiguity. 549 550 551----- Calendrical issues ----- 552 553Calendrical issues are a bit out of scope for a time zone database, 554but they indicate the sort of problems that we would run into if we 555extended the time zone database further into the past. An excellent 556resource in this area is Nachum Dershowitz and Edward M. Reingold, 557Calendrical Calculations: Third Edition, Cambridge University Press (2008) 558<http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>. 559Other information and sources are given below. They sometimes disagree. 560 561 562France 563 564Gregorian calendar adopted 1582-12-20. 565French Revolutionary calendar used 1793-11-24 through 1805-12-31, 566and (in Paris only) 1871-05-06 through 1871-05-23. 567 568 569Russia 570 571From Chris Carrier (1996-12-02): 572On 1929-10-01 the Soviet Union instituted an "Eternal Calendar" 573with 30-day months plus 5 holidays, with a 5-day week. 574On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 575Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 576reverted to the 7-day week. With the 6-day week the usual days 577off were the 6th, 12th, 18th, 24th and 30th of the month. 578(Source: Evitiar Zerubavel, _The Seven Day Circle_) 579 580 581Mark Brader reported a similar story in "The Book of Calendars", edited 582by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 583 584From: Petteri Sulonen (via Usenet) 585Date: 14 Jan 1999 00:00:00 GMT 586... 587 588If your source is correct, how come documents between 1929 and 1940 were 589still dated using the conventional, Gregorian calendar? 590 591I can post a scan of a document dated December 1, 1934, signed by 592Yenukidze, the secretary, on behalf of Kalinin, the President of the 593Executive Committee of the Supreme Soviet, if you like. 594 595 596 597Sweden (and Finland) 598 599From: Mark Brader 600Subject: Re: Gregorian reform - a part of locale? 601<news:1996Jul6.012937.29190@sq.com> 602Date: 1996-07-06 603 604In 1700, Denmark made the transition from Julian to Gregorian. Sweden 605decided to *start* a transition in 1700 as well, but rather than have one of 606those unsightly calendar gaps :-), they simply decreed that the next leap 607year after 1696 would be in 1744 - putting the whole country on a calendar 608different from both Julian and Gregorian for a period of 40 years. 609 610However, in 1704 something went wrong and the plan was not carried through; 611they did, after all, have a leap year that year. And one in 1708. In 1712 612they gave it up and went back to Julian, putting 30 days in February that 613year!... 614 615Then in 1753, Sweden made the transition to Gregorian in the usual manner, 616getting there only 13 years behind the original schedule. 617 618(A previous posting of this story was challenged, and Swedish readers 619produced the following references to support it: "Tider��kning och historia" 620by Natanael Beckman (1924) and "Tid, en bok om tider��kning och 621kalenderv��sen" by Lars-Olof Lod��n (1968). 622 623 624Grotefend's data 625 626From: "Michael Palmer" [with one obvious typo fixed] 627Subject: Re: Gregorian Calendar (was Re: Another FHC related question 628Newsgroups: soc.genealogy.german 629Date: Tue, 9 Feb 1999 02:32:48 -800 630... 631 632The following is a(n incomplete) listing, arranged chronologically, of 633European states, with the date they converted from the Julian to the 634Gregorian calendar: 635 63604/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 637 Catholics and Danzig only) 63809/20 Dec 1582 - France, Lorraine 639 64021 Dec 1582/ 641 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 64210/21 Feb 1583 - bishopric of Liege (L��ttich) 64313/24 Feb 1583 - bishopric of Augsburg 64404/15 Oct 1583 - electorate of Trier 64505/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 646 Salzburg, Brixen 64713/24 Oct 1583 - Austrian Oberelsa�� and Breisgau 64820/31 Oct 1583 - bishopric of Basel 64902/13 Nov 1583 - duchy of J��lich-Berg 65002/13 Nov 1583 - electorate and city of K��ln 65104/15 Nov 1583 - bishopric of W��rzburg 65211/22 Nov 1583 - electorate of Mainz 65316/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 65417/28 Nov 1583 - bishopric of M��nster and duchy of Cleve 65514/25 Dec 1583 - Steiermark 656 65706/17 Jan 1584 - Austria and Bohemia 65811/22 Jan 1584 - Lucerne, Uri, Schwyz, Zug, Freiburg, Solothurn 65912/23 Jan 1584 - Silesia and the Lausitz 66022 Jan/ 661 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 662 Jun 1584 - Unterwalden 66301/12 Jul 1584 - duchy of Westfalen 664 66516/27 Jun 1585 - bishopric of Paderborn 666 66714/25 Dec 1590 - Transylvania 668 66922 Aug/ 670 02 Sep 1612 - duchy of Prussia 671 67213/24 Dec 1614 - Pfalz-Neuburg 673 674 1617 - duchy of Kurland (reverted to the Julian calendar in 675 1796) 676 677 1624 - bishopric of Osnabr��ck 678 679 1630 - bishopric of Minden 680 68115/26 Mar 1631 - bishopric of Hildesheim 682 683 1655 - Kanton Wallis 684 68505/16 Feb 1682 - city of Strassburg 686 68718 Feb/ 688 01 Mar 1700 - Protestant Germany (including Swedish possessions in 689 Germany), Denmark, Norway 69030 Jun/ 691 12 Jul 1700 - Gelderland, Zutphen 69210 Nov/ 693 12 Dec 1700 - Utrecht, Overijssel 694 69531 Dec 1700/ 696 12 Jan 1701 - Friesland, Groningen, Z��rich, Bern, Basel, Geneva, 697 Turgau, and Schaffhausen 698 699 1724 - Glarus, Appenzell, and the city of St. Gallen 700 70101 Jan 1750 - Pisa and Florence 702 70302/14 Sep 1752 - Great Britain 704 70517 Feb/ 706 01 Mar 1753 - Sweden 707 7081760-1812 - Graub��nden 709 710The Russian empire (including Finland and the Baltic states) did not 711convert to the Gregorian calendar until the Soviet revolution of 1917. 712 713Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 714Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 715(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 716 717 718----- Time and time zones on Mars ----- 719 720Some people have adjusted their work schedules to fit Mars time. 721Dozens of special Mars watches were built for Jet Propulsion 722Laboratory workers who kept Mars time during the Mars Exploration 723Rovers mission (2004). These timepieces look like normal Seikos and 724Citizens but use Mars seconds rather than terrestrial seconds. 725 726A Mars solar day is called a "sol" and has a mean period equal to 727about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 728divided into a conventional 24-hour clock, so each Mars second equals 729about 1.02749125 terrestrial seconds. 730 731The prime meridian of Mars goes through the center of the crater 732Airy-0, named in honor of the British astronomer who built the 733Greenwich telescope that defines Earth's prime meridian. Mean solar 734time on the Mars prime meridian is called Mars Coordinated Time (MTC). 735 736Each landed mission on Mars has adopted a different reference for 737solar time keeping, so there is no real standard for Mars time zones. 738For example, the Mars Exploration Rover project (2004) defined two 739time zones "Local Solar Time A" and "Local Solar Time B" for its two 740missions, each zone designed so that its time equals local true solar 741time at approximately the middle of the nominal mission. Such a "time 742zone" is not particularly suited for any application other than the 743mission itself. 744 745Many calendars have been proposed for Mars, but none have achieved 746wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 747sequential count of Mars solar days elapsed since about 1873-12-29 74812:00 GMT. 749 750The tz database does not currently support Mars time, but it is 751documented here in the hopes that support will be added eventually. 752 753Sources: 754 755Michael Allison and Robert Schmunk, 756"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" 757<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08). 758 759Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times 760<http://articles.latimes.com/2004/jan/14/science/sci-marstime14> 761(2004-01-14), pp A1, A20-A21. 762 763 764----- 765Local Variables: 766coding: utf-8 767End: 768