Theory revision 1.19
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 three or more characters that are ASCII alphanumerics or '+' or '-'. 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 lower-case letters are now allowed. 185 Also, POSIX from 2001 on relaxed the rule to allow '-', '+', 186 and alphanumeric characters from the portable character set 187 in the current locale. In practice ASCII alphanumerics and 188 '+' and '-' are safe in all locales. 189 190 In other words, in the C locale the POSIX extended regular 191 expression [-+[:alnum:]]{3,} should match the abbreviation. 192 This guarantees that all abbreviations could have been 193 specified by a POSIX TZ string. 194 195 Use abbreviations that are in common use among English-speakers, 196 e.g. 'EST' for Eastern Standard Time in North America. 197 We assume that applications translate them to other languages 198 as part of the normal localization process; for example, 199 a French application might translate 'EST' to 'HNE'. 200 201 For zones whose times are taken from a city's longitude, use the 202 traditional xMT notation, e.g. 'PMT' for Paris Mean Time. 203 The only name like this in current use is 'GMT'. 204 205 Use 'LMT' for local mean time of locations before the introduction 206 of standard time; see "Scope of the tz database". 207 208 If there is no common English abbreviation, use numeric offsets like 209 -05 and +0830 that are generated by zic's %z notation. 210 211 [The remaining guidelines predate the introduction of %z. 212 They are problematic as they mean tz data entries invent 213 notation rather than record it. These guidelines are now 214 deprecated and the plan is to gradually move to %z for 215 inhabited locations and to "-00" for uninhabited locations.] 216 217 If there is no common English abbreviation, abbreviate the English 218 translation of the usual phrase used by native speakers. 219 If this is not available or is a phrase mentioning the country 220 (e.g. "Cape Verde Time"), then: 221 222 When a country is identified with a single or principal zone, 223 append 'T' to the country's ISO code, e.g. 'CVT' for 224 Cape Verde Time. For summer time append 'ST'; 225 for double summer time append 'DST'; etc. 226 Otherwise, take the first three letters of an English place 227 name identifying each zone and append 'T', 'ST', etc. 228 as before; e.g. 'VLAST' for VLAdivostok Summer Time. 229 230 Use UT (with time zone abbreviation 'zzz') for locations while 231 uninhabited. The 'zzz' mnemonic is that these locations are, 232 in some sense, asleep. 233 234Application writers should note that these abbreviations are ambiguous 235in practice: e.g. 'CST' has a different meaning in China than 236it does in the United States. In new applications, it's often better 237to use numeric UT offsets like '-0600' instead of time zone 238abbreviations like 'CST'; this avoids the ambiguity. 239 240 241----- Accuracy of the tz database ----- 242 243The tz database is not authoritative, and it surely has errors. 244Corrections are welcome and encouraged; see the file CONTRIBUTING. 245Users requiring authoritative data should consult national standards 246bodies and the references cited in the database's comments. 247 248Errors in the tz database arise from many sources: 249 250 * The tz database predicts future time stamps, and current predictions 251 will be incorrect after future governments change the rules. 252 For example, if today someone schedules a meeting for 13:00 next 253 October 1, Casablanca time, and tomorrow Morocco changes its 254 daylight saving rules, software can mess up after the rule change 255 if it blithely relies on conversions made before the change. 256 257 * The pre-1970 entries in this database cover only a tiny sliver of how 258 clocks actually behaved; the vast majority of the necessary 259 information was lost or never recorded. Thousands more zones would 260 be needed if the tz database's scope were extended to cover even 261 just the known or guessed history of standard time; for example, 262 the current single entry for France would need to split into dozens 263 of entries, perhaps hundreds. And in most of the world even this 264 approach would be misleading due to widespread disagreement or 265 indifference about what times should be observed. In her 2015 book 266 "The Global Transformation of Time, 1870-1950", Vanessa Ogle writes 267 "Outside of Europe and North America there was no system of time 268 zones at all, often not even a stable landscape of mean times, 269 prior to the middle decades of the twentieth century". See: 270 Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17 271 https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle 272 273 * Most of the pre-1970 data entries come from unreliable sources, often 274 astrology books that lack citations and whose compilers evidently 275 invented entries when the true facts were unknown, without 276 reporting which entries were known and which were invented. 277 These books often contradict each other or give implausible entries, 278 and on the rare occasions when they are checked they are 279 typically found to be incorrect. 280 281 * For the UK the tz database relies on years of first-class work done by 282 Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>. 283 Other countries are not done nearly as well. 284 285 * Sometimes, different people in the same city would maintain clocks 286 that differed significantly. Railway time was used by railroad 287 companies (which did not always agree with each other), 288 church-clock time was used for birth certificates, etc. 289 Often this was merely common practice, but sometimes it was set by law. 290 For example, from 1891 to 1911 the UT offset in France was legally 291 0:09:21 outside train stations and 0:04:21 inside. 292 293 * Although a named location in the tz database stands for the 294 containing region, its pre-1970 data entries are often accurate for 295 only a small subset of that region. For example, Europe/London 296 stands for the United Kingdom, but its pre-1847 times are valid 297 only for locations that have London's exact meridian, and its 1847 298 transition to GMT is known to be valid only for the L&NW and the 299 Caledonian railways. 300 301 * The tz database does not record the earliest time for which a zone's 302 data entries are thereafter valid for every location in the region. 303 For example, Europe/London is valid for all locations in its 304 region after GMT was made the standard time, but the date of 305 standardization (1880-08-02) is not in the tz database, other than 306 in commentary. For many zones the earliest time of validity is 307 unknown. 308 309 * The tz database does not record a region's boundaries, and in many 310 cases the boundaries are not known. For example, the zone 311 America/Kentucky/Louisville represents a region around the city of 312 Louisville, the boundaries of which are unclear. 313 314 * Changes that are modeled as instantaneous transitions in the tz 315 database were often spread out over hours, days, or even decades. 316 317 * Even if the time is specified by law, locations sometimes 318 deliberately flout the law. 319 320 * Early timekeeping practices, even assuming perfect clocks, were 321 often not specified to the accuracy that the tz database requires. 322 323 * Sometimes historical timekeeping was specified more precisely 324 than what the tz database can handle. For example, from 1909 to 325 1937 Netherlands clocks were legally UT+00:19:32.13, but the tz 326 database cannot represent the fractional second. 327 328 * Even when all the timestamp transitions recorded by the tz database 329 are correct, the tz rules that generate them may not faithfully 330 reflect the historical rules. For example, from 1922 until World 331 War II the UK moved clocks forward the day following the third 332 Saturday in April unless that was Easter, in which case it moved 333 clocks forward the previous Sunday. Because the tz database has no 334 way to specify Easter, these exceptional years are entered as 335 separate tz Rule lines, even though the legal rules did not change. 336 337 * The tz database models pre-standard time using the proleptic Gregorian 338 calendar and local mean time (LMT), but many people used other 339 calendars and other timescales. For example, the Roman Empire used 340 the Julian calendar, and had 12 varying-length daytime hours with a 341 non-hour-based system at night. 342 343 * Early clocks were less reliable, and data entries do not represent 344 this unreliability. 345 346 * As for leap seconds, civil time was not based on atomic time before 347 1972, and we don't know the history of earth's rotation accurately 348 enough to map SI seconds to historical solar time to more than 349 about one-hour accuracy. See: Morrison LV, Stephenson FR. 350 Historical values of the Earth's clock error Delta T and the 351 calculation of eclipses. J Hist Astron. 2004;35:327-36 352 <http://adsabs.harvard.edu/full/2004JHA....35..327M>; 353 Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 354 <http://adsabs.harvard.edu/full/2005JHA....36..339M>. 355 356 * The relationship between POSIX time (that is, UTC but ignoring leap 357 seconds) and UTC is not agreed upon after 1972. Although the POSIX 358 clock officially stops during an inserted leap second, at least one 359 proposed standard has it jumping back a second instead; and in 360 practice POSIX clocks more typically either progress glacially during 361 a leap second, or are slightly slowed while near a leap second. 362 363 * The tz database does not represent how uncertain its information is. 364 Ideally it would contain information about when data entries are 365 incomplete or dicey. Partial temporal knowledge is a field of 366 active research, though, and it's not clear how to apply it here. 367 368In short, many, perhaps most, of the tz database's pre-1970 and future 369time stamps are either wrong or misleading. Any attempt to pass the 370tz database off as the definition of time should be unacceptable to 371anybody who cares about the facts. In particular, the tz database's 372LMT offsets should not be considered meaningful, and should not prompt 373creation of zones merely because two locations differ in LMT or 374transitioned to standard time at different dates. 375 376 377----- Time and date functions ----- 378 379The tz code contains time and date functions that are upwards 380compatible with those of POSIX. 381 382POSIX has the following properties and limitations. 383 384* In POSIX, time display in a process is controlled by the 385 environment variable TZ. Unfortunately, the POSIX TZ string takes 386 a form that is hard to describe and is error-prone in practice. 387 Also, POSIX TZ strings can't deal with other (for example, Israeli) 388 daylight saving time rules, or situations where more than two 389 time zone abbreviations are used in an area. 390 391 The POSIX TZ string takes the following form: 392 393 stdoffset[dst[offset][,date[/time],date[/time]]] 394 395 where: 396 397 std and dst 398 are 3 or more characters specifying the standard 399 and daylight saving time (DST) zone names. 400 Starting with POSIX.1-2001, std and dst may also be 401 in a quoted form like "<UTC+10>"; this allows 402 "+" and "-" in the names. 403 offset 404 is of the form '[+-]hh:[mm[:ss]]' and specifies the 405 offset west of UT. 'hh' may be a single digit; 0<=hh<=24. 406 The default DST offset is one hour ahead of standard time. 407 date[/time],date[/time] 408 specifies the beginning and end of DST. If this is absent, 409 the system supplies its own rules for DST, and these can 410 differ from year to year; typically US DST rules are used. 411 time 412 takes the form 'hh:[mm[:ss]]' and defaults to 02:00. 413 This is the same format as the offset, except that a 414 leading '+' or '-' is not allowed. 415 date 416 takes one of the following forms: 417 Jn (1<=n<=365) 418 origin-1 day number not counting February 29 419 n (0<=n<=365) 420 origin-0 day number counting February 29 if present 421 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 422 for the dth day of week n of month m of the year, 423 where week 1 is the first week in which day d appears, 424 and '5' stands for the last week in which day d appears 425 (which may be either the 4th or 5th week). 426 Typically, this is the only useful form; 427 the n and Jn forms are rarely used. 428 429 Here is an example POSIX TZ string, for US Pacific time using rules 430 appropriate from 1987 through 2006: 431 432 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00' 433 434 This POSIX TZ string is hard to remember, and mishandles time stamps 435 before 1987 and after 2006. With this package you can use this 436 instead: 437 438 TZ='America/Los_Angeles' 439 440* POSIX does not define the exact meaning of TZ values like "EST5EDT". 441 Typically the current US DST rules are used to interpret such values, 442 but this means that the US DST rules are compiled into each program 443 that does time conversion. This means that when US time conversion 444 rules change (as in the United States in 1987), all programs that 445 do time conversion must be recompiled to ensure proper results. 446 447* In POSIX, there's no tamper-proof way for a process to learn the 448 system's best idea of local wall clock. (This is important for 449 applications that an administrator wants used only at certain times - 450 without regard to whether the user has fiddled the "TZ" environment 451 variable. While an administrator can "do everything in UTC" to get 452 around the problem, doing so is inconvenient and precludes handling 453 daylight saving time shifts - as might be required to limit phone 454 calls to off-peak hours.) 455 456* POSIX requires that systems ignore leap seconds. 457 458* The tz code attempts to support all the time_t implementations 459 allowed by POSIX. The time_t type represents a nonnegative count of 460 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 461 In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit 462 signed time_t values stop working after 2038-01-19 03:14:07 UTC, so 463 new implementations these days typically use a signed 64-bit integer. 464 Unsigned 32-bit integers are used on one or two platforms, 465 and 36-bit and 40-bit integers are also used occasionally. 466 Although earlier POSIX versions allowed time_t to be a 467 floating-point type, this was not supported by any practical 468 systems, and POSIX.1-2013 and the tz code both require time_t 469 to be an integer type. 470 471These are the extensions that have been made to the POSIX functions: 472 473* The "TZ" environment variable is used in generating the name of a file 474 from which time zone information is read (or is interpreted a la 475 POSIX); "TZ" is no longer constrained to be a three-letter time zone 476 name followed by a number of hours and an optional three-letter 477 daylight time zone name. The daylight saving time rules to be used 478 for a particular time zone are encoded in the time zone file; 479 the format of the file allows U.S., Australian, and other rules to be 480 encoded, and allows for situations where more than two time zone 481 abbreviations are used. 482 483 It was recognized that allowing the "TZ" environment variable to 484 take on values such as "America/New_York" might cause "old" programs 485 (that expect "TZ" to have a certain form) to operate incorrectly; 486 consideration was given to using some other environment variable 487 (for example, "TIMEZONE") to hold the string used to generate the 488 time zone information file name. In the end, however, it was decided 489 to continue using "TZ": it is widely used for time zone purposes; 490 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 491 and systems where "new" forms of "TZ" might cause problems can simply 492 use TZ values such as "EST5EDT" which can be used both by 493 "new" programs (a la POSIX) and "old" programs (as zone names and 494 offsets). 495 496* To handle places where more than two time zone abbreviations are used, 497 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 498 (where "tmp" is the value the function returns) to the time zone 499 abbreviation to be used. This differs from POSIX, where the elements 500 of tzname are only changed as a result of calls to tzset. 501 502* Since the "TZ" environment variable can now be used to control time 503 conversion, the "daylight" and "timezone" variables are no longer 504 needed. (These variables are defined and set by "tzset"; however, their 505 values will not be used by "localtime.") 506 507* The "localtime" function has been set up to deliver correct results 508 for near-minimum or near-maximum time_t values. (A comment in the 509 source code tells how to get compatibly wrong results). 510 511* A function "tzsetwall" has been added to arrange for the system's 512 best approximation to local wall clock time to be delivered by 513 subsequent calls to "localtime." Source code for portable 514 applications that "must" run on local wall clock time should call 515 "tzsetwall();" if such code is moved to "old" systems that don't 516 provide tzsetwall, you won't be able to generate an executable program. 517 (These time zone functions also arrange for local wall clock time to be 518 used if tzset is called - directly or indirectly - and there's no "TZ" 519 environment variable; portable applications should not, however, rely 520 on this behavior since it's not the way SVR2 systems behave.) 521 522* Negative time_t values are supported, on systems where time_t is signed. 523 524* These functions can account for leap seconds, thanks to Bradley White. 525 526Points of interest to folks with other systems: 527 528* This package is already part of many POSIX-compliant hosts, 529 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 530 On such hosts, the primary use of this package 531 is to update obsolete time zone rule tables. 532 To do this, you may need to compile the time zone compiler 533 'zic' supplied with this package instead of using the system 'zic', 534 since the format of zic's input changed slightly in late 1994, 535 and many vendors still do not support the new input format. 536 537* The UNIX Version 7 "timezone" function is not present in this package; 538 it's impossible to reliably map timezone's arguments (a "minutes west 539 of GMT" value and a "daylight saving time in effect" flag) to a 540 time zone abbreviation, and we refuse to guess. 541 Programs that in the past used the timezone function may now examine 542 tzname[localtime(&clock)->tm_isdst] to learn the correct time 543 zone abbreviation to use. Alternatively, use 544 localtime(&clock)->tm_zone if this has been enabled. 545 546* The 4.2BSD gettimeofday function is not used in this package. 547 This formerly let users obtain the current UTC offset and DST flag, 548 but this functionality was removed in later versions of BSD. 549 550* In SVR2, time conversion fails for near-minimum or near-maximum 551 time_t values when doing conversions for places that don't use UT. 552 This package takes care to do these conversions correctly. 553 554The functions that are conditionally compiled if STD_INSPIRED is defined 555should, at this point, be looked on primarily as food for thought. They are 556not in any sense "standard compatible" - some are not, in fact, specified in 557*any* standard. They do, however, represent responses of various authors to 558standardization proposals. 559 560Other time conversion proposals, in particular the one developed by folks at 561Hewlett Packard, offer a wider selection of functions that provide capabilities 562beyond those provided here. The absence of such functions from this package 563is not meant to discourage the development, standardization, or use of such 564functions. Rather, their absence reflects the decision to make this package 565contain valid extensions to POSIX, to ensure its broad acceptability. If 566more powerful time conversion functions can be standardized, so much the 567better. 568 569 570----- Calendrical issues ----- 571 572Calendrical issues are a bit out of scope for a time zone database, 573but they indicate the sort of problems that we would run into if we 574extended the time zone database further into the past. An excellent 575resource in this area is Nachum Dershowitz and Edward M. Reingold, 576Calendrical Calculations: Third Edition, Cambridge University Press (2008) 577<http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>. 578Other information and sources are given below. They sometimes disagree. 579 580 581France 582 583Gregorian calendar adopted 1582-12-20. 584French Revolutionary calendar used 1793-11-24 through 1805-12-31, 585and (in Paris only) 1871-05-06 through 1871-05-23. 586 587 588Russia 589 590From Chris Carrier (1996-12-02): 591On 1929-10-01 the Soviet Union instituted an "Eternal Calendar" 592with 30-day months plus 5 holidays, with a 5-day week. 593On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 594Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 595reverted to the 7-day week. With the 6-day week the usual days 596off were the 6th, 12th, 18th, 24th and 30th of the month. 597(Source: Evitiar Zerubavel, _The Seven Day Circle_) 598 599 600Mark Brader reported a similar story in "The Book of Calendars", edited 601by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 602 603From: Petteri Sulonen (via Usenet) 604Date: 14 Jan 1999 00:00:00 GMT 605... 606 607If your source is correct, how come documents between 1929 and 1940 were 608still dated using the conventional, Gregorian calendar? 609 610I can post a scan of a document dated December 1, 1934, signed by 611Yenukidze, the secretary, on behalf of Kalinin, the President of the 612Executive Committee of the Supreme Soviet, if you like. 613 614 615 616Sweden (and Finland) 617 618From: Mark Brader 619Subject: Re: Gregorian reform - a part of locale? 620<news:1996Jul6.012937.29190@sq.com> 621Date: 1996-07-06 622 623In 1700, Denmark made the transition from Julian to Gregorian. Sweden 624decided to *start* a transition in 1700 as well, but rather than have one of 625those unsightly calendar gaps :-), they simply decreed that the next leap 626year after 1696 would be in 1744 - putting the whole country on a calendar 627different from both Julian and Gregorian for a period of 40 years. 628 629However, in 1704 something went wrong and the plan was not carried through; 630they did, after all, have a leap year that year. And one in 1708. In 1712 631they gave it up and went back to Julian, putting 30 days in February that 632year!... 633 634Then in 1753, Sweden made the transition to Gregorian in the usual manner, 635getting there only 13 years behind the original schedule. 636 637(A previous posting of this story was challenged, and Swedish readers 638produced the following references to support it: "Tider��kning och historia" 639by Natanael Beckman (1924) and "Tid, en bok om tider��kning och 640kalenderv��sen" by Lars-Olof Lod��n (1968). 641 642 643Grotefend's data 644 645From: "Michael Palmer" [with one obvious typo fixed] 646Subject: Re: Gregorian Calendar (was Re: Another FHC related question 647Newsgroups: soc.genealogy.german 648Date: Tue, 9 Feb 1999 02:32:48 -800 649... 650 651The following is a(n incomplete) listing, arranged chronologically, of 652European states, with the date they converted from the Julian to the 653Gregorian calendar: 654 65504/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 656 Catholics and Danzig only) 65709/20 Dec 1582 - France, Lorraine 658 65921 Dec 1582/ 660 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 66110/21 Feb 1583 - bishopric of Liege (L��ttich) 66213/24 Feb 1583 - bishopric of Augsburg 66304/15 Oct 1583 - electorate of Trier 66405/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 665 Salzburg, Brixen 66613/24 Oct 1583 - Austrian Oberelsa�� and Breisgau 66720/31 Oct 1583 - bishopric of Basel 66802/13 Nov 1583 - duchy of J��lich-Berg 66902/13 Nov 1583 - electorate and city of K��ln 67004/15 Nov 1583 - bishopric of W��rzburg 67111/22 Nov 1583 - electorate of Mainz 67216/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 67317/28 Nov 1583 - bishopric of M��nster and duchy of Cleve 67414/25 Dec 1583 - Steiermark 675 67606/17 Jan 1584 - Austria and Bohemia 67711/22 Jan 1584 - Lucerne, Uri, Schwyz, Zug, Freiburg, Solothurn 67812/23 Jan 1584 - Silesia and the Lausitz 67922 Jan/ 680 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 681 Jun 1584 - Unterwalden 68201/12 Jul 1584 - duchy of Westfalen 683 68416/27 Jun 1585 - bishopric of Paderborn 685 68614/25 Dec 1590 - Transylvania 687 68822 Aug/ 689 02 Sep 1612 - duchy of Prussia 690 69113/24 Dec 1614 - Pfalz-Neuburg 692 693 1617 - duchy of Kurland (reverted to the Julian calendar in 694 1796) 695 696 1624 - bishopric of Osnabr��ck 697 698 1630 - bishopric of Minden 699 70015/26 Mar 1631 - bishopric of Hildesheim 701 702 1655 - Kanton Wallis 703 70405/16 Feb 1682 - city of Strassburg 705 70618 Feb/ 707 01 Mar 1700 - Protestant Germany (including Swedish possessions in 708 Germany), Denmark, Norway 70930 Jun/ 710 12 Jul 1700 - Gelderland, Zutphen 71110 Nov/ 712 12 Dec 1700 - Utrecht, Overijssel 713 71431 Dec 1700/ 715 12 Jan 1701 - Friesland, Groningen, Z��rich, Bern, Basel, Geneva, 716 Turgau, and Schaffhausen 717 718 1724 - Glarus, Appenzell, and the city of St. Gallen 719 72001 Jan 1750 - Pisa and Florence 721 72202/14 Sep 1752 - Great Britain 723 72417 Feb/ 725 01 Mar 1753 - Sweden 726 7271760-1812 - Graub��nden 728 729The Russian empire (including Finland and the Baltic states) did not 730convert to the Gregorian calendar until the Soviet revolution of 1917. 731 732Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 733Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 734(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 735 736 737----- Time and time zones on Mars ----- 738 739Some people's work schedules use Mars time. Jet Propulsion Laboratory 740(JPL) coordinators have kept Mars time on and off at least since 1997 741for the Mars Pathfinder mission. Some of their family members have 742also adapted to Mars time. Dozens of special Mars watches were built 743for JPL workers who kept Mars time during the Mars Exploration 744Rovers mission (2004). These timepieces look like normal Seikos and 745Citizens but use Mars seconds rather than terrestrial seconds. 746 747A Mars solar day is called a "sol" and has a mean period equal to 748about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 749divided into a conventional 24-hour clock, so each Mars second equals 750about 1.02749125 terrestrial seconds. 751 752The prime meridian of Mars goes through the center of the crater 753Airy-0, named in honor of the British astronomer who built the 754Greenwich telescope that defines Earth's prime meridian. Mean solar 755time on the Mars prime meridian is called Mars Coordinated Time (MTC). 756 757Each landed mission on Mars has adopted a different reference for 758solar time keeping, so there is no real standard for Mars time zones. 759For example, the Mars Exploration Rover project (2004) defined two 760time zones "Local Solar Time A" and "Local Solar Time B" for its two 761missions, each zone designed so that its time equals local true solar 762time at approximately the middle of the nominal mission. Such a "time 763zone" is not particularly suited for any application other than the 764mission itself. 765 766Many calendars have been proposed for Mars, but none have achieved 767wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 768sequential count of Mars solar days elapsed since about 1873-12-29 76912:00 GMT. 770 771The tz database does not currently support Mars time, but it is 772documented here in the hopes that support will be added eventually. 773 774Sources: 775 776Michael Allison and Robert Schmunk, 777"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" 778<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08). 779 780Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times 781<http://articles.latimes.com/2004/jan/14/science/sci-marstime14> 782(2004-01-14), pp A1, A20-A21. 783 784Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26) 785<http://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/> 786 787----- 788 789This file is in the public domain, so clarified as of 2009-05-17 by 790Arthur David Olson. 791 792----- 793Local Variables: 794coding: utf-8 795End: 796