Theory revision 1.18
1Theory and pragmatics of the tz code and data 2 3 4----- Outline ----- 5 6 Scope of the tz database 7 Names of time zone rules 8 Time zone abbreviations 9 Accuracy of the tz database 10 Time and date functions 11 Calendrical issues 12 Time and time zones on Mars 13 14 15----- Scope of the tz database ----- 16 17The tz database attempts to record the history and predicted future of 18all computer-based clocks that track civil time. To represent this 19data, the world is partitioned into regions whose clocks all agree 20about time stamps that occur after the somewhat-arbitrary cutoff point 21of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, 22the database records all known clock transitions, and labels the region 23with a notable location. Although 1970 is a somewhat-arbitrary 24cutoff, there are significant challenges to moving the cutoff earlier 25even by a decade or two, due to the wide variety of local practices 26before computer timekeeping became prevalent. 27 28Clock transitions before 1970 are recorded for each such location, 29because most systems support time stamps before 1970 and could 30misbehave if data entries were omitted for pre-1970 transitions. 31However, the database is not designed for and does not suffice for 32applications requiring accurate handling of all past times everywhere, 33as it would take far too much effort and guesswork to record all 34details of pre-1970 civil timekeeping. 35 36As described below, reference source code for using the tz database is 37also available. The tz code is upwards compatible with POSIX, an 38international standard for UNIX-like systems. As of this writing, the 39current edition of POSIX is: 40 41 The Open Group Base Specifications Issue 7 42 IEEE Std 1003.1, 2013 Edition 43 <http://pubs.opengroup.org/onlinepubs/9699919799/> 44 45 46 47----- Names of time zone rules ----- 48 49Each of the database's time zone rules has a unique name. 50Inexperienced users are not expected to select these names unaided. 51Distributors should provide documentation and/or a simple selection 52interface that explains the names; for one example, see the 'tzselect' 53program in the tz code. The Unicode Common Locale Data Repository 54<http://cldr.unicode.org/> contains data that may be useful for other 55selection interfaces. 56 57The time zone rule naming conventions attempt to strike a balance 58among the following goals: 59 60 * Uniquely identify every region where clocks have agreed since 1970. 61 This is essential for the intended use: static clocks keeping local 62 civil time. 63 64 * Indicate to experts where that region is. 65 66 * Be robust in the presence of political changes. For example, names 67 of countries are ordinarily not used, to avoid incompatibilities 68 when countries change their name (e.g. Zaire->Congo) or when 69 locations change countries (e.g. Hong Kong from UK colony to 70 China). 71 72 * Be portable to a wide variety of implementations. 73 74 * Use a consistent naming conventions over the entire world. 75 76Names normally have the form AREA/LOCATION, where AREA is the name 77of a continent or ocean, and LOCATION is the name of a specific 78location within that region. North and South America share the same 79area, 'America'. Typical names are 'Africa/Cairo', 'America/New_York', 80and 'Pacific/Honolulu'. 81 82Here are the general rules used for choosing location names, 83in decreasing order of importance: 84 85 Use only valid POSIX file name components (i.e., the parts of 86 names other than '/'). Do not use the file name 87 components '.' and '..'. Within a file name component, 88 use only ASCII letters, '.', '-' and '_'. Do not use 89 digits, as that might create an ambiguity with POSIX 90 TZ strings. A file name component must not exceed 14 91 characters or start with '-'. E.g., prefer 'Brunei' 92 to 'Bandar_Seri_Begawan'. Exceptions: see the discussion 93 of legacy names below. 94 A name must not be empty, or contain '//', or start or end with '/'. 95 Do not use names that differ only in case. Although the reference 96 implementation is case-sensitive, some other implementations 97 are not, and they would mishandle names differing only in case. 98 If one name A is an initial prefix of another name AB (ignoring case), 99 then B must not start with '/', as a regular file cannot have 100 the same name as a directory in POSIX. For example, 101 'America/New_York' precludes 'America/New_York/Bronx'. 102 Uninhabited regions like the North Pole and Bouvet Island 103 do not need locations, since local time is not defined there. 104 There should typically be at least one name for each ISO 3166-1 105 officially assigned two-letter code for an inhabited country 106 or territory. 107 If all the clocks in a region have agreed since 1970, 108 don't bother to include more than one location 109 even if subregions' clocks disagreed before 1970. 110 Otherwise these tables would become annoyingly large. 111 If a name is ambiguous, use a less ambiguous alternative; 112 e.g. many cities are named San Jos�� and Georgetown, so 113 prefer 'Costa_Rica' to 'San_Jose' and 'Guyana' to 'Georgetown'. 114 Keep locations compact. Use cities or small islands, not countries 115 or regions, so that any future time zone changes do not split 116 locations into different time zones. E.g. prefer 'Paris' 117 to 'France', since France has had multiple time zones. 118 Use mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and 119 prefer 'Athens' to the Greek '����������' or the Romanized 'Ath��na'. 120 The POSIX file name restrictions encourage this rule. 121 Use the most populous among locations in a zone, 122 e.g. prefer 'Shanghai' to 'Beijing'. Among locations with 123 similar populations, pick the best-known location, 124 e.g. prefer 'Rome' to 'Milan'. 125 Use the singular form, e.g. prefer 'Canary' to 'Canaries'. 126 Omit common suffixes like '_Islands' and '_City', unless that 127 would lead to ambiguity. E.g. prefer 'Cayman' to 128 'Cayman_Islands' and 'Guatemala' to 'Guatemala_City', 129 but prefer 'Mexico_City' to 'Mexico' because the country 130 of Mexico has several time zones. 131 Use '_' to represent a space. 132 Omit '.' from abbreviations in names, e.g. prefer 'St_Helena' 133 to 'St._Helena'. 134 Do not change established names if they only marginally 135 violate the above rules. For example, don't change 136 the existing name 'Rome' to 'Milan' merely because 137 Milan's population has grown to be somewhat greater 138 than Rome's. 139 If a name is changed, put its old spelling in the 'backward' file. 140 This means old spellings will continue to work. 141 142The file 'zone1970.tab' lists geographical locations used to name time 143zone rules. It is intended to be an exhaustive list of names for 144geographic regions as described above; this is a subset of the names 145in the data. Although a 'zone1970.tab' location's longitude 146corresponds to its LMT offset with one hour for every 15 degrees east 147longitude, this relationship is not exact. 148 149Older versions of this package used a different naming scheme, 150and these older names are still supported. 151See the file 'backward' for most of these older names 152(e.g., 'US/Eastern' instead of 'America/New_York'). 153The other old-fashioned names still supported are 154'WET', 'CET', 'MET', and 'EET' (see the file 'europe'). 155 156Older versions of this package defined legacy names that are 157incompatible with the first rule of location names, but which are 158still supported. These legacy names are mostly defined in the file 159'etcetera'. Also, the file 'backward' defines the legacy names 160'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file 161'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT', 162'MST7MDT', and 'PST8PDT'. 163 164Excluding 'backward' should not affect the other data. If 165'backward' is excluded, excluding 'etcetera' should not affect the 166remaining data. 167 168 169----- Time zone abbreviations ----- 170 171When this package is installed, it generates time zone abbreviations 172like 'EST' to be compatible with human tradition and POSIX. 173Here are the general rules used for choosing time zone abbreviations, 174in decreasing order of importance: 175 176 Use abbreviations that consist of three or more ASCII letters. 177 Previous editions of this database also used characters like 178 ' ' and '?', but these characters have a special meaning to 179 the shell and cause commands like 180 set `date` 181 to have unexpected effects. 182 Previous editions of this rule required upper-case letters, 183 but the Congressman who introduced Chamorro Standard Time 184 preferred "ChST", so the rule has been relaxed. 185 186 This rule guarantees that all abbreviations could have 187 been specified by a POSIX TZ string. POSIX 188 requires at least three characters for an 189 abbreviation. POSIX through 2000 says that an abbreviation 190 cannot start with ':', and cannot contain ',', '-', 191 '+', NUL, or a digit. POSIX from 2001 on changes this 192 rule to say that an abbreviation can contain only '-', '+', 193 and alphanumeric characters from the portable character set 194 in the current locale. To be portable to both sets of 195 rules, an abbreviation must therefore use only ASCII 196 letters. 197 198 Use abbreviations that are in common use among English-speakers, 199 e.g. 'EST' for Eastern Standard Time in North America. 200 We assume that applications translate them to other languages 201 as part of the normal localization process; for example, 202 a French application might translate 'EST' to 'HNE'. 203 204 For zones whose times are taken from a city's longitude, use the 205 traditional xMT notation, e.g. 'PMT' for Paris Mean Time. 206 The only name like this in current use is 'GMT'. 207 208 Use 'LMT' for local mean time of locations before the introduction 209 of standard time; see "Scope of the tz database". 210 211 If there is no common English abbreviation, use numeric offsets like 212 -05 and +0830 that are generated by zic's %z notation. 213 214 [The remaining guidelines predate the introduction of %z. 215 They are problematic as they mean tz data entries invent 216 notation rather than record it. These guidelines are now 217 deprecated and the plan is to gradually move to %z for 218 inhabited locations and to "-00" for uninhabited locations.] 219 220 If there is no common English abbreviation, abbreviate the English 221 translation of the usual phrase used by native speakers. 222 If this is not available or is a phrase mentioning the country 223 (e.g. "Cape Verde Time"), then: 224 225 When a country is identified with a single or principal zone, 226 append 'T' to the country's ISO code, e.g. 'CVT' for 227 Cape Verde Time. For summer time append 'ST'; 228 for double summer time append 'DST'; etc. 229 Otherwise, take the first three letters of an English place 230 name identifying each zone and append 'T', 'ST', etc. 231 as before; e.g. 'VLAST' for VLAdivostok Summer Time. 232 233 Use UT (with time zone abbreviation 'zzz') for locations while 234 uninhabited. The 'zzz' mnemonic is that these locations are, 235 in some sense, asleep. 236 237Application writers should note that these abbreviations are ambiguous 238in practice: e.g. 'CST' has a different meaning in China than 239it does in the United States. In new applications, it's often better 240to use numeric UT offsets like '-0600' instead of time zone 241abbreviations like 'CST'; this avoids the ambiguity. 242 243 244----- Accuracy of the tz database ----- 245 246The tz database is not authoritative, and it surely has errors. 247Corrections are welcome and encouraged; see the file CONTRIBUTING. 248Users requiring authoritative data should consult national standards 249bodies and the references cited in the database's comments. 250 251Errors in the tz database arise from many sources: 252 253 * The tz database predicts future time stamps, and current predictions 254 will be incorrect after future governments change the rules. 255 For example, if today someone schedules a meeting for 13:00 next 256 October 1, Casablanca time, and tomorrow Morocco changes its 257 daylight saving rules, software can mess up after the rule change 258 if it blithely relies on conversions made before the change. 259 260 * The pre-1970 entries in this database cover only a tiny sliver of how 261 clocks actually behaved; the vast majority of the necessary 262 information was lost or never recorded. Thousands more zones would 263 be needed if the tz database's scope were extended to cover even 264 just the known or guessed history of standard time; for example, 265 the current single entry for France would need to split into dozens 266 of entries, perhaps hundreds. 267 268 * Most of the pre-1970 data entries come from unreliable sources, often 269 astrology books that lack citations and whose compilers evidently 270 invented entries when the true facts were unknown, without 271 reporting which entries were known and which were invented. 272 These books often contradict each other or give implausible entries, 273 and on the rare occasions when they are checked they are 274 typically found to be incorrect. 275 276 * For the UK the tz database relies on years of first-class work done by 277 Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>. 278 Other countries are not done nearly as well. 279 280 * Sometimes, different people in the same city would maintain clocks 281 that differed significantly. Railway time was used by railroad 282 companies (which did not always agree with each other), 283 church-clock time was used for birth certificates, etc. 284 Often this was merely common practice, but sometimes it was set by law. 285 For example, from 1891 to 1911 the UT offset in France was legally 286 0:09:21 outside train stations and 0:04:21 inside. 287 288 * Although a named location in the tz database stands for the 289 containing region, its pre-1970 data entries are often accurate for 290 only a small subset of that region. For example, Europe/London 291 stands for the United Kingdom, but its pre-1847 times are valid 292 only for locations that have London's exact meridian, and its 1847 293 transition to GMT is known to be valid only for the L&NW and the 294 Caledonian railways. 295 296 * The tz database does not record the earliest time for which a zone's 297 data entries are thereafter valid for every location in the region. 298 For example, Europe/London is valid for all locations in its 299 region after GMT was made the standard time, but the date of 300 standardization (1880-08-02) is not in the tz database, other than 301 in commentary. For many zones the earliest time of validity is 302 unknown. 303 304 * The tz database does not record a region's boundaries, and in many 305 cases the boundaries are not known. For example, the zone 306 America/Kentucky/Louisville represents a region around the city of 307 Louisville, the boundaries of which are unclear. 308 309 * Changes that are modeled as instantaneous transitions in the tz 310 database were often spread out over hours, days, or even decades. 311 312 * Even if the time is specified by law, locations sometimes 313 deliberately flout the law. 314 315 * Early timekeeping practices, even assuming perfect clocks, were 316 often not specified to the accuracy that the tz database requires. 317 318 * Sometimes historical timekeeping was specified more precisely 319 than what the tz database can handle. For example, from 1909 to 320 1937 Netherlands clocks were legally UT+00:19:32.13, but the tz 321 database cannot represent the fractional second. 322 323 * Even when all the timestamp transitions recorded by the tz database 324 are correct, the tz rules that generate them may not faithfully 325 reflect the historical rules. For example, from 1922 until World 326 War II the UK moved clocks forward the day following the third 327 Saturday in April unless that was Easter, in which case it moved 328 clocks forward the previous Sunday. Because the tz database has no 329 way to specify Easter, these exceptional years are entered as 330 separate tz Rule lines, even though the legal rules did not change. 331 332 * The tz database models pre-standard time using the proleptic Gregorian 333 calendar and local mean time (LMT), but many people used other 334 calendars and other timescales. For example, the Roman Empire used 335 the Julian calendar, and had 12 varying-length daytime hours with a 336 non-hour-based system at night. 337 338 * Early clocks were less reliable, and data entries do not represent 339 this unreliability. 340 341 * As for leap seconds, civil time was not based on atomic time before 342 1972, and we don't know the history of earth's rotation accurately 343 enough to map SI seconds to historical solar time to more than 344 about one-hour accuracy. See: Morrison LV, Stephenson FR. 345 Historical values of the Earth's clock error Delta T and the 346 calculation of eclipses. J Hist Astron. 2004;35:327-36 347 <http://adsabs.harvard.edu/full/2004JHA....35..327M>; 348 Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 349 <http://adsabs.harvard.edu/full/2005JHA....36..339M>. 350 351 * The relationship between POSIX time (that is, UTC but ignoring leap 352 seconds) and UTC is not agreed upon after 1972. Although the POSIX 353 clock officially stops during an inserted leap second, at least one 354 proposed standard has it jumping back a second instead; and in 355 practice POSIX clocks more typically either progress glacially during 356 a leap second, or are slightly slowed while near a leap second. 357 358 * The tz database does not represent how uncertain its information is. 359 Ideally it would contain information about when data entries are 360 incomplete or dicey. Partial temporal knowledge is a field of 361 active research, though, and it's not clear how to apply it here. 362 363In short, many, perhaps most, of the tz database's pre-1970 and future 364time stamps are either wrong or misleading. Any attempt to pass the 365tz database off as the definition of time should be unacceptable to 366anybody who cares about the facts. In particular, the tz database's 367LMT offsets should not be considered meaningful, and should not prompt 368creation of zones merely because two locations differ in LMT or 369transitioned to standard time at different dates. 370 371 372----- Time and date functions ----- 373 374The tz code contains time and date functions that are upwards 375compatible with those of POSIX. 376 377POSIX has the following properties and limitations. 378 379* In POSIX, time display in a process is controlled by the 380 environment variable TZ. Unfortunately, the POSIX TZ string takes 381 a form that is hard to describe and is error-prone in practice. 382 Also, POSIX TZ strings can't deal with other (for example, Israeli) 383 daylight saving time rules, or situations where more than two 384 time zone abbreviations are used in an area. 385 386 The POSIX TZ string takes the following form: 387 388 stdoffset[dst[offset][,date[/time],date[/time]]] 389 390 where: 391 392 std and dst 393 are 3 or more characters specifying the standard 394 and daylight saving time (DST) zone names. 395 Starting with POSIX.1-2001, std and dst may also be 396 in a quoted form like "<UTC+10>"; this allows 397 "+" and "-" in the names. 398 offset 399 is of the form '[+-]hh:[mm[:ss]]' and specifies the 400 offset west of UT. 'hh' may be a single digit; 0<=hh<=24. 401 The default DST offset is one hour ahead of standard time. 402 date[/time],date[/time] 403 specifies the beginning and end of DST. If this is absent, 404 the system supplies its own rules for DST, and these can 405 differ from year to year; typically US DST rules are used. 406 time 407 takes the form 'hh:[mm[:ss]]' and defaults to 02:00. 408 This is the same format as the offset, except that a 409 leading '+' or '-' is not allowed. 410 date 411 takes one of the following forms: 412 Jn (1<=n<=365) 413 origin-1 day number not counting February 29 414 n (0<=n<=365) 415 origin-0 day number counting February 29 if present 416 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 417 for the dth day of week n of month m of the year, 418 where week 1 is the first week in which day d appears, 419 and '5' stands for the last week in which day d appears 420 (which may be either the 4th or 5th week). 421 Typically, this is the only useful form; 422 the n and Jn forms are rarely used. 423 424 Here is an example POSIX TZ string, for US Pacific time using rules 425 appropriate from 1987 through 2006: 426 427 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00' 428 429 This POSIX TZ string is hard to remember, and mishandles time stamps 430 before 1987 and after 2006. With this package you can use this 431 instead: 432 433 TZ='America/Los_Angeles' 434 435* POSIX does not define the exact meaning of TZ values like "EST5EDT". 436 Typically the current US DST rules are used to interpret such values, 437 but this means that the US DST rules are compiled into each program 438 that does time conversion. This means that when US time conversion 439 rules change (as in the United States in 1987), all programs that 440 do time conversion must be recompiled to ensure proper results. 441 442* In POSIX, there's no tamper-proof way for a process to learn the 443 system's best idea of local wall clock. (This is important for 444 applications that an administrator wants used only at certain times - 445 without regard to whether the user has fiddled the "TZ" environment 446 variable. While an administrator can "do everything in UTC" to get 447 around the problem, doing so is inconvenient and precludes handling 448 daylight saving time shifts - as might be required to limit phone 449 calls to off-peak hours.) 450 451* POSIX requires that systems ignore leap seconds. 452 453* The tz code attempts to support all the time_t implementations 454 allowed by POSIX. The time_t type represents a nonnegative count of 455 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 456 In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit 457 signed time_t values stop working after 2038-01-19 03:14:07 UTC, so 458 new implementations these days typically use a signed 64-bit integer. 459 Unsigned 32-bit integers are used on one or two platforms, 460 and 36-bit and 40-bit integers are also used occasionally. 461 Although earlier POSIX versions allowed time_t to be a 462 floating-point type, this was not supported by any practical 463 systems, and POSIX.1-2013 and the tz code both require time_t 464 to be an integer type. 465 466These are the extensions that have been made to the POSIX functions: 467 468* The "TZ" environment variable is used in generating the name of a file 469 from which time zone information is read (or is interpreted a la 470 POSIX); "TZ" is no longer constrained to be a three-letter time zone 471 name followed by a number of hours and an optional three-letter 472 daylight time zone name. The daylight saving time rules to be used 473 for a particular time zone are encoded in the time zone file; 474 the format of the file allows U.S., Australian, and other rules to be 475 encoded, and allows for situations where more than two time zone 476 abbreviations are used. 477 478 It was recognized that allowing the "TZ" environment variable to 479 take on values such as "America/New_York" might cause "old" programs 480 (that expect "TZ" to have a certain form) to operate incorrectly; 481 consideration was given to using some other environment variable 482 (for example, "TIMEZONE") to hold the string used to generate the 483 time zone information file name. In the end, however, it was decided 484 to continue using "TZ": it is widely used for time zone purposes; 485 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 486 and systems where "new" forms of "TZ" might cause problems can simply 487 use TZ values such as "EST5EDT" which can be used both by 488 "new" programs (a la POSIX) and "old" programs (as zone names and 489 offsets). 490 491* To handle places where more than two time zone abbreviations are used, 492 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 493 (where "tmp" is the value the function returns) to the time zone 494 abbreviation to be used. This differs from POSIX, where the elements 495 of tzname are only changed as a result of calls to tzset. 496 497* Since the "TZ" environment variable can now be used to control time 498 conversion, the "daylight" and "timezone" variables are no longer 499 needed. (These variables are defined and set by "tzset"; however, their 500 values will not be used by "localtime.") 501 502* The "localtime" function has been set up to deliver correct results 503 for near-minimum or near-maximum time_t values. (A comment in the 504 source code tells how to get compatibly wrong results). 505 506* A function "tzsetwall" has been added to arrange for the system's 507 best approximation to local wall clock time to be delivered by 508 subsequent calls to "localtime." Source code for portable 509 applications that "must" run on local wall clock time should call 510 "tzsetwall();" if such code is moved to "old" systems that don't 511 provide tzsetwall, you won't be able to generate an executable program. 512 (These time zone functions also arrange for local wall clock time to be 513 used if tzset is called - directly or indirectly - and there's no "TZ" 514 environment variable; portable applications should not, however, rely 515 on this behavior since it's not the way SVR2 systems behave.) 516 517* Negative time_t values are supported, on systems where time_t is signed. 518 519* These functions can account for leap seconds, thanks to Bradley White. 520 521Points of interest to folks with other systems: 522 523* This package is already part of many POSIX-compliant hosts, 524 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 525 On such hosts, the primary use of this package 526 is to update obsolete time zone rule tables. 527 To do this, you may need to compile the time zone compiler 528 'zic' supplied with this package instead of using the system 'zic', 529 since the format of zic's input changed slightly in late 1994, 530 and many vendors still do not support the new input format. 531 532* The UNIX Version 7 "timezone" function is not present in this package; 533 it's impossible to reliably map timezone's arguments (a "minutes west 534 of GMT" value and a "daylight saving time in effect" flag) to a 535 time zone abbreviation, and we refuse to guess. 536 Programs that in the past used the timezone function may now examine 537 tzname[localtime(&clock)->tm_isdst] to learn the correct time 538 zone abbreviation to use. Alternatively, use 539 localtime(&clock)->tm_zone if this has been enabled. 540 541* The 4.2BSD gettimeofday function is not used in this package. 542 This formerly let users obtain the current UTC offset and DST flag, 543 but this functionality was removed in later versions of BSD. 544 545* In SVR2, time conversion fails for near-minimum or near-maximum 546 time_t values when doing conversions for places that don't use UT. 547 This package takes care to do these conversions correctly. 548 549The functions that are conditionally compiled if STD_INSPIRED is defined 550should, at this point, be looked on primarily as food for thought. They are 551not in any sense "standard compatible" - some are not, in fact, specified in 552*any* standard. They do, however, represent responses of various authors to 553standardization proposals. 554 555Other time conversion proposals, in particular the one developed by folks at 556Hewlett Packard, offer a wider selection of functions that provide capabilities 557beyond those provided here. The absence of such functions from this package 558is not meant to discourage the development, standardization, or use of such 559functions. Rather, their absence reflects the decision to make this package 560contain valid extensions to POSIX, to ensure its broad acceptability. If 561more powerful time conversion functions can be standardized, so much the 562better. 563 564 565----- Calendrical issues ----- 566 567Calendrical issues are a bit out of scope for a time zone database, 568but they indicate the sort of problems that we would run into if we 569extended the time zone database further into the past. An excellent 570resource in this area is Nachum Dershowitz and Edward M. Reingold, 571Calendrical Calculations: Third Edition, Cambridge University Press (2008) 572<http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>. 573Other information and sources are given below. They sometimes disagree. 574 575 576France 577 578Gregorian calendar adopted 1582-12-20. 579French Revolutionary calendar used 1793-11-24 through 1805-12-31, 580and (in Paris only) 1871-05-06 through 1871-05-23. 581 582 583Russia 584 585From Chris Carrier (1996-12-02): 586On 1929-10-01 the Soviet Union instituted an "Eternal Calendar" 587with 30-day months plus 5 holidays, with a 5-day week. 588On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 589Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 590reverted to the 7-day week. With the 6-day week the usual days 591off were the 6th, 12th, 18th, 24th and 30th of the month. 592(Source: Evitiar Zerubavel, _The Seven Day Circle_) 593 594 595Mark Brader reported a similar story in "The Book of Calendars", edited 596by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 597 598From: Petteri Sulonen (via Usenet) 599Date: 14 Jan 1999 00:00:00 GMT 600... 601 602If your source is correct, how come documents between 1929 and 1940 were 603still dated using the conventional, Gregorian calendar? 604 605I can post a scan of a document dated December 1, 1934, signed by 606Yenukidze, the secretary, on behalf of Kalinin, the President of the 607Executive Committee of the Supreme Soviet, if you like. 608 609 610 611Sweden (and Finland) 612 613From: Mark Brader 614Subject: Re: Gregorian reform - a part of locale? 615<news:1996Jul6.012937.29190@sq.com> 616Date: 1996-07-06 617 618In 1700, Denmark made the transition from Julian to Gregorian. Sweden 619decided to *start* a transition in 1700 as well, but rather than have one of 620those unsightly calendar gaps :-), they simply decreed that the next leap 621year after 1696 would be in 1744 - putting the whole country on a calendar 622different from both Julian and Gregorian for a period of 40 years. 623 624However, in 1704 something went wrong and the plan was not carried through; 625they did, after all, have a leap year that year. And one in 1708. In 1712 626they gave it up and went back to Julian, putting 30 days in February that 627year!... 628 629Then in 1753, Sweden made the transition to Gregorian in the usual manner, 630getting there only 13 years behind the original schedule. 631 632(A previous posting of this story was challenged, and Swedish readers 633produced the following references to support it: "Tider��kning och historia" 634by Natanael Beckman (1924) and "Tid, en bok om tider��kning och 635kalenderv��sen" by Lars-Olof Lod��n (1968). 636 637 638Grotefend's data 639 640From: "Michael Palmer" [with one obvious typo fixed] 641Subject: Re: Gregorian Calendar (was Re: Another FHC related question 642Newsgroups: soc.genealogy.german 643Date: Tue, 9 Feb 1999 02:32:48 -800 644... 645 646The following is a(n incomplete) listing, arranged chronologically, of 647European states, with the date they converted from the Julian to the 648Gregorian calendar: 649 65004/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 651 Catholics and Danzig only) 65209/20 Dec 1582 - France, Lorraine 653 65421 Dec 1582/ 655 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 65610/21 Feb 1583 - bishopric of Liege (L��ttich) 65713/24 Feb 1583 - bishopric of Augsburg 65804/15 Oct 1583 - electorate of Trier 65905/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 660 Salzburg, Brixen 66113/24 Oct 1583 - Austrian Oberelsa�� and Breisgau 66220/31 Oct 1583 - bishopric of Basel 66302/13 Nov 1583 - duchy of J��lich-Berg 66402/13 Nov 1583 - electorate and city of K��ln 66504/15 Nov 1583 - bishopric of W��rzburg 66611/22 Nov 1583 - electorate of Mainz 66716/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 66817/28 Nov 1583 - bishopric of M��nster and duchy of Cleve 66914/25 Dec 1583 - Steiermark 670 67106/17 Jan 1584 - Austria and Bohemia 67211/22 Jan 1584 - Lucerne, Uri, Schwyz, Zug, Freiburg, Solothurn 67312/23 Jan 1584 - Silesia and the Lausitz 67422 Jan/ 675 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 676 Jun 1584 - Unterwalden 67701/12 Jul 1584 - duchy of Westfalen 678 67916/27 Jun 1585 - bishopric of Paderborn 680 68114/25 Dec 1590 - Transylvania 682 68322 Aug/ 684 02 Sep 1612 - duchy of Prussia 685 68613/24 Dec 1614 - Pfalz-Neuburg 687 688 1617 - duchy of Kurland (reverted to the Julian calendar in 689 1796) 690 691 1624 - bishopric of Osnabr��ck 692 693 1630 - bishopric of Minden 694 69515/26 Mar 1631 - bishopric of Hildesheim 696 697 1655 - Kanton Wallis 698 69905/16 Feb 1682 - city of Strassburg 700 70118 Feb/ 702 01 Mar 1700 - Protestant Germany (including Swedish possessions in 703 Germany), Denmark, Norway 70430 Jun/ 705 12 Jul 1700 - Gelderland, Zutphen 70610 Nov/ 707 12 Dec 1700 - Utrecht, Overijssel 708 70931 Dec 1700/ 710 12 Jan 1701 - Friesland, Groningen, Z��rich, Bern, Basel, Geneva, 711 Turgau, and Schaffhausen 712 713 1724 - Glarus, Appenzell, and the city of St. Gallen 714 71501 Jan 1750 - Pisa and Florence 716 71702/14 Sep 1752 - Great Britain 718 71917 Feb/ 720 01 Mar 1753 - Sweden 721 7221760-1812 - Graub��nden 723 724The Russian empire (including Finland and the Baltic states) did not 725convert to the Gregorian calendar until the Soviet revolution of 1917. 726 727Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 728Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 729(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 730 731 732----- Time and time zones on Mars ----- 733 734Some people's work schedules use Mars time. Jet Propulsion Laboratory 735(JPL) coordinators have kept Mars time on and off at least since 1997 736for the Mars Pathfinder mission. Some of their family members have 737also adapted to Mars time. Dozens of special Mars watches were built 738for JPL workers who kept Mars time during the Mars Exploration 739Rovers mission (2004). These timepieces look like normal Seikos and 740Citizens but use Mars seconds rather than terrestrial seconds. 741 742A Mars solar day is called a "sol" and has a mean period equal to 743about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 744divided into a conventional 24-hour clock, so each Mars second equals 745about 1.02749125 terrestrial seconds. 746 747The prime meridian of Mars goes through the center of the crater 748Airy-0, named in honor of the British astronomer who built the 749Greenwich telescope that defines Earth's prime meridian. Mean solar 750time on the Mars prime meridian is called Mars Coordinated Time (MTC). 751 752Each landed mission on Mars has adopted a different reference for 753solar time keeping, so there is no real standard for Mars time zones. 754For example, the Mars Exploration Rover project (2004) defined two 755time zones "Local Solar Time A" and "Local Solar Time B" for its two 756missions, each zone designed so that its time equals local true solar 757time at approximately the middle of the nominal mission. Such a "time 758zone" is not particularly suited for any application other than the 759mission itself. 760 761Many calendars have been proposed for Mars, but none have achieved 762wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 763sequential count of Mars solar days elapsed since about 1873-12-29 76412:00 GMT. 765 766The tz database does not currently support Mars time, but it is 767documented here in the hopes that support will be added eventually. 768 769Sources: 770 771Michael Allison and Robert Schmunk, 772"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" 773<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08). 774 775Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times 776<http://articles.latimes.com/2004/jan/14/science/sci-marstime14> 777(2004-01-14), pp A1, A20-A21. 778 779Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26) 780<http://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/> 781 782----- 783 784This file is in the public domain, so clarified as of 2009-05-17 by 785Arthur David Olson. 786 787----- 788Local Variables: 789coding: utf-8 790End: 791