Theory revision 1.21
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 '-00') for locations while 231 uninhabited. The leading '-' is a flag that the time 232 zone is in some sense undefined; this notation is 233 derived from Internet RFC 3339. 234 235Application writers should note that these abbreviations are ambiguous 236in practice: e.g. 'CST' has a different meaning in China than 237it does in the United States. In new applications, it's often better 238to use numeric UT offsets like '-0600' instead of time zone 239abbreviations like 'CST'; this avoids the ambiguity. 240 241 242----- Accuracy of the tz database ----- 243 244The tz database is not authoritative, and it surely has errors. 245Corrections are welcome and encouraged; see the file CONTRIBUTING. 246Users requiring authoritative data should consult national standards 247bodies and the references cited in the database's comments. 248 249Errors in the tz database arise from many sources: 250 251 * The tz database predicts future time stamps, and current predictions 252 will be incorrect after future governments change the rules. 253 For example, if today someone schedules a meeting for 13:00 next 254 October 1, Casablanca time, and tomorrow Morocco changes its 255 daylight saving rules, software can mess up after the rule change 256 if it blithely relies on conversions made before the change. 257 258 * The pre-1970 entries in this database cover only a tiny sliver of how 259 clocks actually behaved; the vast majority of the necessary 260 information was lost or never recorded. Thousands more zones would 261 be needed if the tz database's scope were extended to cover even 262 just the known or guessed history of standard time; for example, 263 the current single entry for France would need to split into dozens 264 of entries, perhaps hundreds. And in most of the world even this 265 approach would be misleading due to widespread disagreement or 266 indifference about what times should be observed. In her 2015 book 267 "The Global Transformation of Time, 1870-1950", Vanessa Ogle writes 268 "Outside of Europe and North America there was no system of time 269 zones at all, often not even a stable landscape of mean times, 270 prior to the middle decades of the twentieth century". See: 271 Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17 272 https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle 273 274 * Most of the pre-1970 data entries come from unreliable sources, often 275 astrology books that lack citations and whose compilers evidently 276 invented entries when the true facts were unknown, without 277 reporting which entries were known and which were invented. 278 These books often contradict each other or give implausible entries, 279 and on the rare occasions when they are checked they are 280 typically found to be incorrect. 281 282 * For the UK the tz database relies on years of first-class work done by 283 Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>. 284 Other countries are not done nearly as well. 285 286 * Sometimes, different people in the same city would maintain clocks 287 that differed significantly. Railway time was used by railroad 288 companies (which did not always agree with each other), 289 church-clock time was used for birth certificates, etc. 290 Often this was merely common practice, but sometimes it was set by law. 291 For example, from 1891 to 1911 the UT offset in France was legally 292 0:09:21 outside train stations and 0:04:21 inside. 293 294 * Although a named location in the tz database stands for the 295 containing region, its pre-1970 data entries are often accurate for 296 only a small subset of that region. For example, Europe/London 297 stands for the United Kingdom, but its pre-1847 times are valid 298 only for locations that have London's exact meridian, and its 1847 299 transition to GMT is known to be valid only for the L&NW and the 300 Caledonian railways. 301 302 * The tz database does not record the earliest time for which a zone's 303 data entries are thereafter valid for every location in the region. 304 For example, Europe/London is valid for all locations in its 305 region after GMT was made the standard time, but the date of 306 standardization (1880-08-02) is not in the tz database, other than 307 in commentary. For many zones the earliest time of validity is 308 unknown. 309 310 * The tz database does not record a region's boundaries, and in many 311 cases the boundaries are not known. For example, the zone 312 America/Kentucky/Louisville represents a region around the city of 313 Louisville, the boundaries of which are unclear. 314 315 * Changes that are modeled as instantaneous transitions in the tz 316 database were often spread out over hours, days, or even decades. 317 318 * Even if the time is specified by law, locations sometimes 319 deliberately flout the law. 320 321 * Early timekeeping practices, even assuming perfect clocks, were 322 often not specified to the accuracy that the tz database requires. 323 324 * Sometimes historical timekeeping was specified more precisely 325 than what the tz database can handle. For example, from 1909 to 326 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz 327 database cannot represent the fractional second. 328 329 * Even when all the timestamp transitions recorded by the tz database 330 are correct, the tz rules that generate them may not faithfully 331 reflect the historical rules. For example, from 1922 until World 332 War II the UK moved clocks forward the day following the third 333 Saturday in April unless that was Easter, in which case it moved 334 clocks forward the previous Sunday. Because the tz database has no 335 way to specify Easter, these exceptional years are entered as 336 separate tz Rule lines, even though the legal rules did not change. 337 338 * The tz database models pre-standard time using the proleptic Gregorian 339 calendar and local mean time (LMT), but many people used other 340 calendars and other timescales. For example, the Roman Empire used 341 the Julian calendar, and had 12 varying-length daytime hours with a 342 non-hour-based system at night. 343 344 * Early clocks were less reliable, and data entries do not represent 345 this unreliability. 346 347 * As for leap seconds, civil time was not based on atomic time before 348 1972, and we don't know the history of earth's rotation accurately 349 enough to map SI seconds to historical solar time to more than 350 about one-hour accuracy. See: Morrison LV, Stephenson FR. 351 Historical values of the Earth's clock error Delta T and the 352 calculation of eclipses. J Hist Astron. 2004;35:327-36 353 <http://adsabs.harvard.edu/full/2004JHA....35..327M>; 354 Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 355 <http://adsabs.harvard.edu/full/2005JHA....36..339M>. 356 357 * The relationship between POSIX time (that is, UTC but ignoring leap 358 seconds) and UTC is not agreed upon after 1972. Although the POSIX 359 clock officially stops during an inserted leap second, at least one 360 proposed standard has it jumping back a second instead; and in 361 practice POSIX clocks more typically either progress glacially during 362 a leap second, or are slightly slowed while near a leap second. 363 364 * The tz database does not represent how uncertain its information is. 365 Ideally it would contain information about when data entries are 366 incomplete or dicey. Partial temporal knowledge is a field of 367 active research, though, and it's not clear how to apply it here. 368 369In short, many, perhaps most, of the tz database's pre-1970 and future 370time stamps are either wrong or misleading. Any attempt to pass the 371tz database off as the definition of time should be unacceptable to 372anybody who cares about the facts. In particular, the tz database's 373LMT offsets should not be considered meaningful, and should not prompt 374creation of zones merely because two locations differ in LMT or 375transitioned to standard time at different dates. 376 377 378----- Time and date functions ----- 379 380The tz code contains time and date functions that are upwards 381compatible with those of POSIX. 382 383POSIX has the following properties and limitations. 384 385* In POSIX, time display in a process is controlled by the 386 environment variable TZ. Unfortunately, the POSIX TZ string takes 387 a form that is hard to describe and is error-prone in practice. 388 Also, POSIX TZ strings can't deal with other (for example, Israeli) 389 daylight saving time rules, or situations where more than two 390 time zone abbreviations are used in an area. 391 392 The POSIX TZ string takes the following form: 393 394 stdoffset[dst[offset][,date[/time],date[/time]]] 395 396 where: 397 398 std and dst 399 are 3 or more characters specifying the standard 400 and daylight saving time (DST) zone names. 401 Starting with POSIX.1-2001, std and dst may also be 402 in a quoted form like "<UTC+10>"; this allows 403 "+" and "-" in the names. 404 offset 405 is of the form '[+-]hh:[mm[:ss]]' and specifies the 406 offset west of UT. 'hh' may be a single digit; 0<=hh<=24. 407 The default DST offset is one hour ahead of standard time. 408 date[/time],date[/time] 409 specifies the beginning and end of DST. If this is absent, 410 the system supplies its own rules for DST, and these can 411 differ from year to year; typically US DST rules are used. 412 time 413 takes the form 'hh:[mm[:ss]]' and defaults to 02:00. 414 This is the same format as the offset, except that a 415 leading '+' or '-' is not allowed. 416 date 417 takes one of the following forms: 418 Jn (1<=n<=365) 419 origin-1 day number not counting February 29 420 n (0<=n<=365) 421 origin-0 day number counting February 29 if present 422 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 423 for the dth day of week n of month m of the year, 424 where week 1 is the first week in which day d appears, 425 and '5' stands for the last week in which day d appears 426 (which may be either the 4th or 5th week). 427 Typically, this is the only useful form; 428 the n and Jn forms are rarely used. 429 430 Here is an example POSIX TZ string, for US Pacific time using rules 431 appropriate from 1987 through 2006: 432 433 TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00' 434 435 This POSIX TZ string is hard to remember, and mishandles time stamps 436 before 1987 and after 2006. With this package you can use this 437 instead: 438 439 TZ='America/Los_Angeles' 440 441* POSIX does not define the exact meaning of TZ values like "EST5EDT". 442 Typically the current US DST rules are used to interpret such values, 443 but this means that the US DST rules are compiled into each program 444 that does time conversion. This means that when US time conversion 445 rules change (as in the United States in 1987), all programs that 446 do time conversion must be recompiled to ensure proper results. 447 448* In POSIX, there's no tamper-proof way for a process to learn the 449 system's best idea of local wall clock. (This is important for 450 applications that an administrator wants used only at certain times - 451 without regard to whether the user has fiddled the "TZ" environment 452 variable. While an administrator can "do everything in UTC" to get 453 around the problem, doing so is inconvenient and precludes handling 454 daylight saving time shifts - as might be required to limit phone 455 calls to off-peak hours.) 456 457* POSIX requires that systems ignore leap seconds. 458 459* The tz code attempts to support all the time_t implementations 460 allowed by POSIX. The time_t type represents a nonnegative count of 461 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 462 In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit 463 signed time_t values stop working after 2038-01-19 03:14:07 UTC, so 464 new implementations these days typically use a signed 64-bit integer. 465 Unsigned 32-bit integers are used on one or two platforms, 466 and 36-bit and 40-bit integers are also used occasionally. 467 Although earlier POSIX versions allowed time_t to be a 468 floating-point type, this was not supported by any practical 469 systems, and POSIX.1-2013 and the tz code both require time_t 470 to be an integer type. 471 472These are the extensions that have been made to the POSIX functions: 473 474* The "TZ" environment variable is used in generating the name of a file 475 from which time zone information is read (or is interpreted a la 476 POSIX); "TZ" is no longer constrained to be a three-letter time zone 477 name followed by a number of hours and an optional three-letter 478 daylight time zone name. The daylight saving time rules to be used 479 for a particular time zone are encoded in the time zone file; 480 the format of the file allows U.S., Australian, and other rules to be 481 encoded, and allows for situations where more than two time zone 482 abbreviations are used. 483 484 It was recognized that allowing the "TZ" environment variable to 485 take on values such as "America/New_York" might cause "old" programs 486 (that expect "TZ" to have a certain form) to operate incorrectly; 487 consideration was given to using some other environment variable 488 (for example, "TIMEZONE") to hold the string used to generate the 489 time zone information file name. In the end, however, it was decided 490 to continue using "TZ": it is widely used for time zone purposes; 491 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 492 and systems where "new" forms of "TZ" might cause problems can simply 493 use TZ values such as "EST5EDT" which can be used both by 494 "new" programs (a la POSIX) and "old" programs (as zone names and 495 offsets). 496 497* To handle places where more than two time zone abbreviations are used, 498 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 499 (where "tmp" is the value the function returns) to the time zone 500 abbreviation to be used. This differs from POSIX, where the elements 501 of tzname are only changed as a result of calls to tzset. 502 503* Since the "TZ" environment variable can now be used to control time 504 conversion, the "daylight" and "timezone" variables are no longer 505 needed. (These variables are defined and set by "tzset"; however, their 506 values will not be used by "localtime.") 507 508* The "localtime" function has been set up to deliver correct results 509 for near-minimum or near-maximum time_t values. (A comment in the 510 source code tells how to get compatibly wrong results). 511 512* A function "tzsetwall" has been added to arrange for the system's 513 best approximation to local wall clock time to be delivered by 514 subsequent calls to "localtime." Source code for portable 515 applications that "must" run on local wall clock time should call 516 "tzsetwall();" if such code is moved to "old" systems that don't 517 provide tzsetwall, you won't be able to generate an executable program. 518 (These time zone functions also arrange for local wall clock time to be 519 used if tzset is called - directly or indirectly - and there's no "TZ" 520 environment variable; portable applications should not, however, rely 521 on this behavior since it's not the way SVR2 systems behave.) 522 523* Negative time_t values are supported, on systems where time_t is signed. 524 525* These functions can account for leap seconds, thanks to Bradley White. 526 527Points of interest to folks with other systems: 528 529* This package is already part of many POSIX-compliant hosts, 530 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 531 On such hosts, the primary use of this package 532 is to update obsolete time zone rule tables. 533 To do this, you may need to compile the time zone compiler 534 'zic' supplied with this package instead of using the system 'zic', 535 since the format of zic's input changed slightly in late 1994, 536 and many vendors still do not support the new input format. 537 538* The UNIX Version 7 "timezone" function is not present in this package; 539 it's impossible to reliably map timezone's arguments (a "minutes west 540 of GMT" value and a "daylight saving time in effect" flag) to a 541 time zone abbreviation, and we refuse to guess. 542 Programs that in the past used the timezone function may now examine 543 tzname[localtime(&clock)->tm_isdst] to learn the correct time 544 zone abbreviation to use. Alternatively, use 545 localtime(&clock)->tm_zone if this has been enabled. 546 547* The 4.2BSD gettimeofday function is not used in this package. 548 This formerly let users obtain the current UTC offset and DST flag, 549 but this functionality was removed in later versions of BSD. 550 551* In SVR2, time conversion fails for near-minimum or near-maximum 552 time_t values when doing conversions for places that don't use UT. 553 This package takes care to do these conversions correctly. 554 555The functions that are conditionally compiled if STD_INSPIRED is defined 556should, at this point, be looked on primarily as food for thought. They are 557not in any sense "standard compatible" - some are not, in fact, specified in 558*any* standard. They do, however, represent responses of various authors to 559standardization proposals. 560 561Other time conversion proposals, in particular the one developed by folks at 562Hewlett Packard, offer a wider selection of functions that provide capabilities 563beyond those provided here. The absence of such functions from this package 564is not meant to discourage the development, standardization, or use of such 565functions. Rather, their absence reflects the decision to make this package 566contain valid extensions to POSIX, to ensure its broad acceptability. If 567more powerful time conversion functions can be standardized, so much the 568better. 569 570 571----- Calendrical issues ----- 572 573Calendrical issues are a bit out of scope for a time zone database, 574but they indicate the sort of problems that we would run into if we 575extended the time zone database further into the past. An excellent 576resource in this area is Nachum Dershowitz and Edward M. Reingold, 577Calendrical Calculations: Third Edition, Cambridge University Press (2008) 578<http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>. 579Other information and sources are given below. They sometimes disagree. 580 581 582France 583 584Gregorian calendar adopted 1582-12-20. 585French Revolutionary calendar used 1793-11-24 through 1805-12-31, 586and (in Paris only) 1871-05-06 through 1871-05-23. 587 588 589Russia 590 591From Chris Carrier (1996-12-02): 592On 1929-10-01 the Soviet Union instituted an "Eternal Calendar" 593with 30-day months plus 5 holidays, with a 5-day week. 594On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 595Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 596reverted to the 7-day week. With the 6-day week the usual days 597off were the 6th, 12th, 18th, 24th and 30th of the month. 598(Source: Evitiar Zerubavel, _The Seven Day Circle_) 599 600 601Mark Brader reported a similar story in "The Book of Calendars", edited 602by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 603 604From: Petteri Sulonen (via Usenet) 605Date: 14 Jan 1999 00:00:00 GMT 606... 607 608If your source is correct, how come documents between 1929 and 1940 were 609still dated using the conventional, Gregorian calendar? 610 611I can post a scan of a document dated December 1, 1934, signed by 612Yenukidze, the secretary, on behalf of Kalinin, the President of the 613Executive Committee of the Supreme Soviet, if you like. 614 615 616 617Sweden (and Finland) 618 619From: Mark Brader 620Subject: Re: Gregorian reform - a part of locale? 621<news:1996Jul6.012937.29190@sq.com> 622Date: 1996-07-06 623 624In 1700, Denmark made the transition from Julian to Gregorian. Sweden 625decided to *start* a transition in 1700 as well, but rather than have one of 626those unsightly calendar gaps :-), they simply decreed that the next leap 627year after 1696 would be in 1744 - putting the whole country on a calendar 628different from both Julian and Gregorian for a period of 40 years. 629 630However, in 1704 something went wrong and the plan was not carried through; 631they did, after all, have a leap year that year. And one in 1708. In 1712 632they gave it up and went back to Julian, putting 30 days in February that 633year!... 634 635Then in 1753, Sweden made the transition to Gregorian in the usual manner, 636getting there only 13 years behind the original schedule. 637 638(A previous posting of this story was challenged, and Swedish readers 639produced the following references to support it: "Tider��kning och historia" 640by Natanael Beckman (1924) and "Tid, en bok om tider��kning och 641kalenderv��sen" by Lars-Olof Lod��n (1968). 642 643 644Grotefend's data 645 646From: "Michael Palmer" [with one obvious typo fixed] 647Subject: Re: Gregorian Calendar (was Re: Another FHC related question 648Newsgroups: soc.genealogy.german 649Date: Tue, 9 Feb 1999 02:32:48 -800 650... 651 652The following is a(n incomplete) listing, arranged chronologically, of 653European states, with the date they converted from the Julian to the 654Gregorian calendar: 655 65604/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 657 Catholics and Danzig only) 65809/20 Dec 1582 - France, Lorraine 659 66021 Dec 1582/ 661 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 66210/21 Feb 1583 - bishopric of Liege (L��ttich) 66313/24 Feb 1583 - bishopric of Augsburg 66404/15 Oct 1583 - electorate of Trier 66505/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 666 Salzburg, Brixen 66713/24 Oct 1583 - Austrian Oberelsa�� and Breisgau 66820/31 Oct 1583 - bishopric of Basel 66902/13 Nov 1583 - duchy of J��lich-Berg 67002/13 Nov 1583 - electorate and city of K��ln 67104/15 Nov 1583 - bishopric of W��rzburg 67211/22 Nov 1583 - electorate of Mainz 67316/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 67417/28 Nov 1583 - bishopric of M��nster and duchy of Cleve 67514/25 Dec 1583 - Steiermark 676 67706/17 Jan 1584 - Austria and Bohemia 67811/22 Jan 1584 - Lucerne, Uri, Schwyz, Zug, Freiburg, Solothurn 67912/23 Jan 1584 - Silesia and the Lausitz 68022 Jan/ 681 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 682 Jun 1584 - Unterwalden 68301/12 Jul 1584 - duchy of Westfalen 684 68516/27 Jun 1585 - bishopric of Paderborn 686 68714/25 Dec 1590 - Transylvania 688 68922 Aug/ 690 02 Sep 1612 - duchy of Prussia 691 69213/24 Dec 1614 - Pfalz-Neuburg 693 694 1617 - duchy of Kurland (reverted to the Julian calendar in 695 1796) 696 697 1624 - bishopric of Osnabr��ck 698 699 1630 - bishopric of Minden 700 70115/26 Mar 1631 - bishopric of Hildesheim 702 703 1655 - Kanton Wallis 704 70505/16 Feb 1682 - city of Strassburg 706 70718 Feb/ 708 01 Mar 1700 - Protestant Germany (including Swedish possessions in 709 Germany), Denmark, Norway 71030 Jun/ 711 12 Jul 1700 - Gelderland, Zutphen 71210 Nov/ 713 12 Dec 1700 - Utrecht, Overijssel 714 71531 Dec 1700/ 716 12 Jan 1701 - Friesland, Groningen, Z��rich, Bern, Basel, Geneva, 717 Turgau, and Schaffhausen 718 719 1724 - Glarus, Appenzell, and the city of St. Gallen 720 72101 Jan 1750 - Pisa and Florence 722 72302/14 Sep 1752 - Great Britain 724 72517 Feb/ 726 01 Mar 1753 - Sweden 727 7281760-1812 - Graub��nden 729 730The Russian empire (including Finland and the Baltic states) did not 731convert to the Gregorian calendar until the Soviet revolution of 1917. 732 733Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 734Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 735(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 736 737 738----- Time and time zones on Mars ----- 739 740Some people's work schedules use Mars time. Jet Propulsion Laboratory 741(JPL) coordinators have kept Mars time on and off at least since 1997 742for the Mars Pathfinder mission. Some of their family members have 743also adapted to Mars time. Dozens of special Mars watches were built 744for JPL workers who kept Mars time during the Mars Exploration 745Rovers mission (2004). These timepieces look like normal Seikos and 746Citizens but use Mars seconds rather than terrestrial seconds. 747 748A Mars solar day is called a "sol" and has a mean period equal to 749about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 750divided into a conventional 24-hour clock, so each Mars second equals 751about 1.02749125 terrestrial seconds. 752 753The prime meridian of Mars goes through the center of the crater 754Airy-0, named in honor of the British astronomer who built the 755Greenwich telescope that defines Earth's prime meridian. Mean solar 756time on the Mars prime meridian is called Mars Coordinated Time (MTC). 757 758Each landed mission on Mars has adopted a different reference for 759solar time keeping, so there is no real standard for Mars time zones. 760For example, the Mars Exploration Rover project (2004) defined two 761time zones "Local Solar Time A" and "Local Solar Time B" for its two 762missions, each zone designed so that its time equals local true solar 763time at approximately the middle of the nominal mission. Such a "time 764zone" is not particularly suited for any application other than the 765mission itself. 766 767Many calendars have been proposed for Mars, but none have achieved 768wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 769sequential count of Mars solar days elapsed since about 1873-12-29 77012:00 GMT. 771 772The tz database does not currently support Mars time, but it is 773documented here in the hopes that support will be added eventually. 774 775Sources: 776 777Michael Allison and Robert Schmunk, 778"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" 779<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08). 780 781Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times 782<http://articles.latimes.com/2004/jan/14/science/sci-marstime14> 783(2004-01-14), pp A1, A20-A21. 784 785Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26) 786<http://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/> 787 788----- 789 790This file is in the public domain, so clarified as of 2009-05-17 by 791Arthur David Olson. 792 793----- 794Local Variables: 795coding: utf-8 796End: 797