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1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <title>Theory and pragmatics of the tz code and data</title> 5 <meta charset="UTF-8"> 6 </head> 7 8 <!-- The somewhat-unusal indenting style in this file is intended to 9 shrink the output of the shell command 'diff Theory Theory.html', 10 where 'Theory' was the plain text file that this file is derived 11 from. The 'Theory' file used leading white space to indent, and 12 when possible that indentation is preserved here. Eventually we 13 may stop doing this and remove this comment. --> 14 15 <body> 16 <h1>Theory and pragmatics of the tz code and data</h1> 17 <h3>Outline</h3> 18 <nav> 19 <ul> 20 <li><a href="#scope">Scope of the tz database</a></li> 21 <li><a href="#naming">Names of time zone rules</a></li> 22 <li><a href="#abbreviations">Time zone abbreviations</a></li> 23 <li><a href="#accuracy">Accuracy of the tz database</a></li> 24 <li><a href="#functions">Time and date functions</a></li> 25 <li><a href="#stability">Interface stability</a></li> 26 <li><a href="#calendar">Calendrical issues</a></li> 27 <li><a href="#planets">Time and time zones on other planets</a></li> 28 </ul> 29 </nav> 30 31 32 <section> 33 <h2 id="scope">Scope of the tz database</h2> 34 <p> 35 The tz database attempts to record the history and predicted future of 36 all computer-based clocks that track civil time. To represent this 37 data, the world is partitioned into regions whose clocks all agree 38 about timestamps that occur after the somewhat-arbitrary cutoff point 39 of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, 40 the database records all known clock transitions, and labels the region 41 with a notable location. Although 1970 is a somewhat-arbitrary 42 cutoff, there are significant challenges to moving the cutoff earlier 43 even by a decade or two, due to the wide variety of local practices 44 before computer timekeeping became prevalent. 45 </p> 46 47 <p> 48 Clock transitions before 1970 are recorded for each such location, 49 because most systems support timestamps before 1970 and could 50 misbehave if data entries were omitted for pre-1970 transitions. 51 However, the database is not designed for and does not suffice for 52 applications requiring accurate handling of all past times everywhere, 53 as it would take far too much effort and guesswork to record all 54 details of pre-1970 civil timekeeping. 55 </p> 56 57 <p> 58 As described below, reference source code for using the tz database is 59 also available. The tz code is upwards compatible with POSIX, an 60 international standard for UNIX-like systems. As of this writing, the 61 current edition of POSIX is: 62 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/"> 63 The Open Group Base Specifications Issue 7</a>, 64 IEEE Std 1003.1-2008, 2016 Edition. 65 </p> 66 </section> 67 68 69 70 <section> 71 <h2 id="naming">Names of time zone rules</h2> 72 <p> 73 Each of the database's time zone rules has a unique name. 74 Inexperienced users are not expected to select these names unaided. 75 Distributors should provide documentation and/or a simple selection 76 interface that explains the names; for one example, see the 'tzselect' 77 program in the tz code. The 78 <a href="http://cldr.unicode.org/">Unicode Common Locale Data 79 Repository</a> contains data that may be useful for other 80 selection interfaces. 81 </p> 82 83 <p> 84 The time zone rule naming conventions attempt to strike a balance 85 among the following goals: 86 </p> 87 <ul> 88 <li> 89 Uniquely identify every region where clocks have agreed since 1970. 90 This is essential for the intended use: static clocks keeping local 91 civil time. 92 </li> 93 <li> 94 Indicate to experts where that region is. 95 </li> 96 <li> 97 Be robust in the presence of political changes. For example, names 98 of countries are ordinarily not used, to avoid incompatibilities 99 when countries change their name (e.g. Zaire→Congo) or when 100 locations change countries (e.g. Hong Kong from UK colony to 101 China). 102 </li> 103 <li> 104 Be portable to a wide variety of implementations. 105 </li> 106 <li> 107 Use a consistent naming conventions over the entire world. 108 </li> 109 </ul> 110 <p> 111 Names normally have the 112 form <var>AREA</var><code>/</code><var>LOCATION</var>, 113 where <var>AREA</var> is the name of a continent or ocean, 114 and <var>LOCATION</var> is the name of a specific 115 location within that region. North and South America share the same 116 area, '<code>America</code>'. Typical names are 117 '<code>Africa/Cairo</code>', '<code>America/New_York</code>', and 118 '<code>Pacific/Honolulu</code>'. 119 </p> 120 121 <p> 122 Here are the general rules used for choosing location names, 123 in decreasing order of importance: 124 </p> 125 <ul> 126 <li> 127 Use only valid POSIX file name components (i.e., the parts of 128 names other than '<code>/</code>'). Do not use the file name 129 components '<code>.</code>' and '<code>..</code>'. 130 Within a file name component, 131 use only ASCII letters, '<code>.</code>', 132 '<code>-</code>' and '<code>_</code>'. Do not use 133 digits, as that might create an ambiguity with POSIX 134 TZ strings. A file name component must not exceed 14 135 characters or start with '<code>-</code>'. E.g., 136 prefer '<code>Brunei</code>' to 137 '<code>Bandar_Seri_Begawan</code>'. Exceptions: see 138 the discussion 139 of legacy names below. 140 </li> 141 <li> 142 A name must not be empty, or contain '<code>//</code>', or 143 start or end with '<code>/</code>'. 144 </li> 145 <li> 146 Do not use names that differ only in case. Although the reference 147 implementation is case-sensitive, some other implementations 148 are not, and they would mishandle names differing only in case. 149 </li> 150 <li> 151 If one name <var>A</var> is an initial prefix of another 152 name <var>AB</var> (ignoring case), then <var>B</var> 153 must not start with '<code>/</code>', as a 154 regular file cannot have 155 the same name as a directory in POSIX. For example, 156 '<code>America/New_York</code>' precludes 157 '<code>America/New_York/Bronx</code>'. 158 </li> 159 <li> 160 Uninhabited regions like the North Pole and Bouvet Island 161 do not need locations, since local time is not defined there. 162 </li> 163 <li> 164 There should typically be at least one name for each ISO 3166-1 165 officially assigned two-letter code for an inhabited country 166 or territory. 167 </li> 168 <li> 169 If all the clocks in a region have agreed since 1970, 170 don't bother to include more than one location 171 even if subregions' clocks disagreed before 1970. 172 Otherwise these tables would become annoyingly large. 173 </li> 174 <li> 175 If a name is ambiguous, use a less ambiguous alternative; 176 e.g. many cities are named San Jos and Georgetown, so 177 prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'. 178 </li> 179 <li> 180 Keep locations compact. Use cities or small islands, not countries 181 or regions, so that any future time zone changes do not split 182 locations into different time zones. E.g. prefer 183 '<code>Paris</code>' to '<code>France</code>', since 184 France has had multiple time zones. 185 </li> 186 <li> 187 Use mainstream English spelling, e.g. prefer 188 '<code>Rome</code>' to '<code>Roma</code>', and prefer 189 '<code>Athens</code>' to the Greek 190 '<code></code>' or the Romanized 191 '<code>Athna</code>'. 192 The POSIX file name restrictions encourage this rule. 193 </li> 194 <li> 195 Use the most populous among locations in a zone, 196 e.g. prefer '<code>Shanghai</code>' to 197 '<code>Beijing</code>'. Among locations with 198 similar populations, pick the best-known location, 199 e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'. 200 </li> 201 <li> 202 Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'. 203 </li> 204 <li> 205 Omit common suffixes like '<code>_Islands</code>' and 206 '<code>_City</code>', unless that would lead to 207 ambiguity. E.g. prefer '<code>Cayman</code>' to 208 '<code>Cayman_Islands</code>' and 209 '<code>Guatemala</code>' to 210 '<code>Guatemala_City</code>', but prefer 211 '<code>Mexico_City</code>' to '<code>Mexico</code>' 212 because the country 213 of Mexico has several time zones. 214 </li> 215 <li> 216 Use '<code>_</code>' to represent a space. 217 </li> 218 <li> 219 Omit '<code>.</code>' from abbreviations in names, e.g. prefer 220 '<code>St_Helena</code>' to '<code>St._Helena</code>'. 221 </li> 222 <li> 223 Do not change established names if they only marginally 224 violate the above rules. For example, don't change 225 the existing name '<code>Rome</code>' to 226 '<code>Milan</code>' merely because 227 Milan's population has grown to be somewhat greater 228 than Rome's. 229 </li> 230 <li> 231 If a name is changed, put its old spelling in the 232 '<code>backward</code>' file. 233 This means old spellings will continue to work. 234 </li> 235 </ul> 236 237 <p> 238 The file '<code>zone1970.tab</code>' lists geographical locations used 239 to name time 240 zone rules. It is intended to be an exhaustive list of names for 241 geographic regions as described above; this is a subset of the names 242 in the data. Although a '<code>zone1970.tab</code>' location's longitude 243 corresponds to its LMT offset with one hour for every 15 degrees east 244 longitude, this relationship is not exact. 245 </p> 246 247 <p> 248 Older versions of this package used a different naming scheme, 249 and these older names are still supported. 250 See the file '<code>backward</code>' for most of these older names 251 (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). 252 The other old-fashioned names still supported are 253 '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>'). 254 </p> 255 256 <p> 257 Older versions of this package defined legacy names that are 258 incompatible with the first rule of location names, but which are 259 still supported. These legacy names are mostly defined in the file 260 '<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names 261 '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the 262 legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'. 263 </p> 264 265 <p> 266 Excluding '<code>backward</code>' should not affect the other data. If 267 '<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the 268 remaining data. 269 </p> 270 271 272 </section> 273 <section> 274 <h2 id="abbreviations">Time zone abbreviations</h2> 275 <p> 276 When this package is installed, it generates time zone abbreviations 277 like '<code>EST</code>' to be compatible with human tradition and POSIX. 278 Here are the general rules used for choosing time zone abbreviations, 279 in decreasing order of importance: 280 <ul> 281 <li> 282 Use three or more characters that are ASCII alphanumerics or 283 '<code>+</code>' or '<code>-</code>'. 284 Previous editions of this database also used characters like 285 '<code> </code>' and '<code>?</code>', but these 286 characters have a special meaning to 287 the shell and cause commands like 288 '<code>set `date`</code>' 289 to have unexpected effects. 290 Previous editions of this rule required upper-case letters, 291 but the Congressman who introduced Chamorro Standard Time 292 preferred "ChST", so lower-case letters are now allowed. 293 Also, POSIX from 2001 on relaxed the rule to allow 294 '<code>-</code>', '<code>+</code>', 295 and alphanumeric characters from the portable character set 296 in the current locale. In practice ASCII alphanumerics and 297 '<code>+</code>' and '<code>-</code>' are safe in all locales. 298 299 In other words, in the C locale the POSIX extended regular 300 expression <code>[-+[:alnum:]]{3,}</code> should match 301 the abbreviation. 302 This guarantees that all abbreviations could have been 303 specified by a POSIX TZ string. 304 </li> 305 <li> 306 Use abbreviations that are in common use among English-speakers, 307 e.g. 'EST' for Eastern Standard Time in North America. 308 We assume that applications translate them to other languages 309 as part of the normal localization process; for example, 310 a French application might translate 'EST' to 'HNE'. 311 </li> 312 <li> 313 For zones whose times are taken from a city's longitude, use the 314 traditional <var>x</var>MT notation, e.g. 'PMT' for 315 Paris Mean Time. 316 The only name like this in current use is 'GMT'. 317 </li> 318 <li> 319 Use 'LMT' for local mean time of locations before the introduction 320 of standard time; see "<a href="#scope">Scope of the 321 tz database</a>". 322 </li> 323 <li> 324 If there is no common English abbreviation, use numeric offsets like 325 <code>-</code>05 and <code>+</code>0830 that are 326 generated by zic's <code>%z</code> notation. 327 </li> 328 <li> 329 Use current abbreviations for older timestamps to avoid confusion. 330 For example, in 1910 a common English abbreviation for UT +01 331 in central Europe was 'MEZ' (short for both "Middle European 332 Zone" and for "Mitteleuropische Zeit" in German). Nowadays 333 'CET' ("Central European Time") is more common in English, and 334 the database uses 'CET' even for circa-1910 timestamps as this 335 is less confusing for modern users and avoids the need for 336 determining when 'CET' supplanted 'MEZ' in common usage. 337 </li> 338 <li> 339 Use a consistent style in a zone's history. For example, if a zone's 340 history tends to use numeric abbreviations and a particular 341 entry could go either way, use a numeric abbreviation. 342 </li> 343 </ul> 344 [The remaining guidelines predate the introduction of <code>%z</code>. 345 They are problematic as they mean tz data entries invent 346 notation rather than record it. These guidelines are now 347 deprecated and the plan is to gradually move to <code>%z</code> for 348 inhabited locations and to "<code>-</code>00" for uninhabited locations.] 349 <ul> 350 <li> 351 If there is no common English abbreviation, abbreviate the English 352 translation of the usual phrase used by native speakers. 353 If this is not available or is a phrase mentioning the country 354 (e.g. "Cape Verde Time"), then: 355 <ul> 356 <li> 357 When a country is identified with a single or principal zone, 358 append 'T' to the country's ISO code, e.g. 'CVT' for 359 Cape Verde Time. For summer time append 'ST'; 360 for double summer time append 'DST'; etc. 361 </li> 362 <li> 363 Otherwise, take the first three letters of an English place 364 name identifying each zone and append 'T', 'ST', etc. 365 as before; e.g. 'CHAST' for CHAtham Summer Time. 366 </li> 367 </ul> 368 </li> 369 <li> 370 Use UT (with time zone abbreviation '<code>-</code>00') for 371 locations while uninhabited. The leading 372 '<code>-</code>' is a flag that the time 373 zone is in some sense undefined; this notation is 374 derived from Internet RFC 3339. 375 </li> 376 </ul> 377 <p> 378 Application writers should note that these abbreviations are ambiguous 379 in practice: e.g. 'CST' has a different meaning in China than 380 it does in the United States. In new applications, it's often better 381 to use numeric UT offsets like '<code>-</code>0600' instead of time zone 382 abbreviations like 'CST'; this avoids the ambiguity. 383 </p> 384 </section> 385 386 387 <section> 388 <h2 id="accuracy">Accuracy of the tz database</h2> 389 <p> 390 The tz database is not authoritative, and it surely has errors. 391 Corrections are welcome and encouraged; see the file CONTRIBUTING. 392 Users requiring authoritative data should consult national standards 393 bodies and the references cited in the database's comments. 394 </p> 395 396 <p> 397 Errors in the tz database arise from many sources: 398 </p> 399 <ul> 400 <li> 401 The tz database predicts future timestamps, and current predictions 402 will be incorrect after future governments change the rules. 403 For example, if today someone schedules a meeting for 13:00 next 404 October 1, Casablanca time, and tomorrow Morocco changes its 405 daylight saving rules, software can mess up after the rule change 406 if it blithely relies on conversions made before the change. 407 </li> 408 <li> 409 The pre-1970 entries in this database cover only a tiny sliver of how 410 clocks actually behaved; the vast majority of the necessary 411 information was lost or never recorded. Thousands more zones would 412 be needed if the tz database's scope were extended to cover even 413 just the known or guessed history of standard time; for example, 414 the current single entry for France would need to split into dozens 415 of entries, perhaps hundreds. And in most of the world even this 416 approach would be misleading due to widespread disagreement or 417 indifference about what times should be observed. In her 2015 book 418 <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes 419 "Outside of Europe and North America there was no system of time 420 zones at all, often not even a stable landscape of mean times, 421 prior to the middle decades of the twentieth century". See: 422 Timothy Shenk, <a 423 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: 424 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. 425 </li> 426 <li> 427 Most of the pre-1970 data entries come from unreliable sources, often 428 astrology books that lack citations and whose compilers evidently 429 invented entries when the true facts were unknown, without 430 reporting which entries were known and which were invented. 431 These books often contradict each other or give implausible entries, 432 and on the rare occasions when they are checked they are 433 typically found to be incorrect. 434 </li> 435 <li> 436 For the UK the tz database relies on years of first-class work done by 437 Joseph Myers and others; see 438 "<a href="https://www.polyomino.org.uk/british-time/">History of 439 legal time in Britain</a>". 440 Other countries are not done nearly as well. 441 </li> 442 <li> 443 Sometimes, different people in the same city would maintain clocks 444 that differed significantly. Railway time was used by railroad 445 companies (which did not always agree with each other), 446 church-clock time was used for birth certificates, etc. 447 Often this was merely common practice, but sometimes it was set by law. 448 For example, from 1891 to 1911 the UT offset in France was legally 449 0:09:21 outside train stations and 0:04:21 inside. 450 </li> 451 <li> 452 Although a named location in the tz database stands for the 453 containing region, its pre-1970 data entries are often accurate for 454 only a small subset of that region. For example, <code>Europe/London</code> 455 stands for the United Kingdom, but its pre-1847 times are valid 456 only for locations that have London's exact meridian, and its 1847 457 transition to GMT is known to be valid only for the L&NW and the 458 Caledonian railways. 459 </li> 460 <li> 461 The tz database does not record the earliest time for which a zone's 462 data entries are thereafter valid for every location in the region. 463 For example, <code>Europe/London</code> is valid for all locations in its 464 region after GMT was made the standard time, but the date of 465 standardization (1880-08-02) is not in the tz database, other than 466 in commentary. For many zones the earliest time of validity is 467 unknown. 468 </li> 469 <li> 470 The tz database does not record a region's boundaries, and in many 471 cases the boundaries are not known. For example, the zone 472 <code>America/Kentucky/Louisville</code> represents a region around 473 the city of 474 Louisville, the boundaries of which are unclear. 475 </li> 476 <li> 477 Changes that are modeled as instantaneous transitions in the tz 478 database were often spread out over hours, days, or even decades. 479 </li> 480 <li> 481 Even if the time is specified by law, locations sometimes 482 deliberately flout the law. 483 </li> 484 <li> 485 Early timekeeping practices, even assuming perfect clocks, were 486 often not specified to the accuracy that the tz database requires. 487 </li> 488 <li> 489 Sometimes historical timekeeping was specified more precisely 490 than what the tz database can handle. For example, from 1909 to 491 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz 492 database cannot represent the fractional second. 493 </li> 494 <li> 495 Even when all the timestamp transitions recorded by the tz database 496 are correct, the tz rules that generate them may not faithfully 497 reflect the historical rules. For example, from 1922 until World 498 War II the UK moved clocks forward the day following the third 499 Saturday in April unless that was Easter, in which case it moved 500 clocks forward the previous Sunday. Because the tz database has no 501 way to specify Easter, these exceptional years are entered as 502 separate tz Rule lines, even though the legal rules did not change. 503 </li> 504 <li> 505 The tz database models pre-standard time using the proleptic Gregorian 506 calendar and local mean time (LMT), but many people used other 507 calendars and other timescales. For example, the Roman Empire used 508 the Julian calendar, and had 12 varying-length daytime hours with a 509 non-hour-based system at night. 510 </li> 511 <li> 512 Early clocks were less reliable, and data entries do not represent 513 clock error. 514 </li> 515 <li> 516 The tz database assumes Universal Time (UT) as an origin, even 517 though UT is not standardized for older timestamps. In the tz 518 database commentary, UT denotes a family of time standards that 519 includes Coordinated Universal Time (UTC) along with other variants 520 such as UT1 and GMT, with days starting at midnight. Although UT 521 equals UTC for modern timestamps, UTC was not defined until 1960, 522 so commentary uses the more-general abbreviation UT for timestamps 523 that might predate 1960. Since UT, UT1, etc. disagree slightly, 524 and since pre-1972 UTC seconds varied in length, interpretation of 525 older timestamps can be problematic when subsecond accuracy is 526 needed. 527 </li> 528 <li> 529 Civil time was not based on atomic time before 1972, and we don't 530 know the history of earth's rotation accurately enough to map SI 531 seconds to historical solar time to more than about one-hour 532 accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY. 533 <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement 534 of the Earth's rotation: 720 BC to AD 2015</a>. 535 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. 536 Also see: Espenak F. <a 537 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty 538 in Delta T (T)</a>. 539 </li> 540 <li> 541 The relationship between POSIX time (that is, UTC but ignoring leap 542 seconds) and UTC is not agreed upon after 1972. Although the POSIX 543 clock officially stops during an inserted leap second, at least one 544 proposed standard has it jumping back a second instead; and in 545 practice POSIX clocks more typically either progress glacially during 546 a leap second, or are slightly slowed while near a leap second. 547 </li> 548 <li> 549 The tz database does not represent how uncertain its information is. 550 Ideally it would contain information about when data entries are 551 incomplete or dicey. Partial temporal knowledge is a field of 552 active research, though, and it's not clear how to apply it here. 553 </li> 554 </ul> 555 <p> 556 In short, many, perhaps most, of the tz database's pre-1970 and future 557 timestamps are either wrong or misleading. Any attempt to pass the 558 tz database off as the definition of time should be unacceptable to 559 anybody who cares about the facts. In particular, the tz database's 560 LMT offsets should not be considered meaningful, and should not prompt 561 creation of zones merely because two locations differ in LMT or 562 transitioned to standard time at different dates. 563 </p> 564 </section> 565 566 567 <section> 568 <h2 id="functions">Time and date functions</h2> 569 <p> 570 The tz code contains time and date functions that are upwards 571 compatible with those of POSIX. 572 </p> 573 574 <p> 575 POSIX has the following properties and limitations. 576 </p> 577 <ul> 578 <li> 579 <p> 580 In POSIX, time display in a process is controlled by the 581 environment variable TZ. Unfortunately, the POSIX TZ string takes 582 a form that is hard to describe and is error-prone in practice. 583 Also, POSIX TZ strings can't deal with other (for example, Israeli) 584 daylight saving time rules, or situations where more than two 585 time zone abbreviations are used in an area. 586 </p> 587 <p> 588 The POSIX TZ string takes the following form: 589 </p> 590 <p> 591 <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]] 592 </p> 593 <p> 594 where: 595 <dl> 596 <dt><var>std</var> and <var>dst</var></dt><dd> 597 are 3 or more characters specifying the standard 598 and daylight saving time (DST) zone names. 599 Starting with POSIX.1-2001, <var>std</var> 600 and <var>dst</var> may also be 601 in a quoted form like '<code><UTC+10></code>'; this allows 602 "<code>+</code>" and "<code>-</code>" in the names. 603 </dd> 604 <dt><var>offset</var></dt><dd> 605 is of the form 606 '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' 607 and specifies the offset west of UT. '<var>hh</var>' 608 may be a single digit; 0≤<var>hh</var>≤24. 609 The default DST offset is one hour ahead of standard time. 610 </dd> 611 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> 612 specifies the beginning and end of DST. If this is absent, 613 the system supplies its own rules for DST, and these can 614 differ from year to year; typically US DST rules are used. 615 </dd> 616 <dt><var>time</var></dt><dd> 617 takes the form 618 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' 619 and defaults to 02:00. 620 This is the same format as the offset, except that a 621 leading '<code>+</code>' or '<code>-</code>' is not allowed. 622 </dd> 623 <dt><var>date</var></dt><dd> 624 takes one of the following forms: 625 <dl> 626 <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> 627 origin-1 day number not counting February 29 628 </dd> 629 <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> 630 origin-0 day number counting February 29 if present 631 </dd> 632 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 1≤<var>m</var>≤12)</dt><dd> 633 for the <var>d</var>th day of 634 week <var>n</var> of month <var>m</var> of the 635 year, where week 1 is the first week in which 636 day <var>d</var> appears, and '<code>5</code>' 637 stands for the last week in which 638 day <var>d</var> appears 639 (which may be either the 4th or 5th week). 640 Typically, this is the only useful form; 641 the <var>n</var> 642 and <code>J</code><var>n</var> forms are 643 rarely used. 644 </dd> 645 </dl> 646 </dd> 647 </dl> 648 Here is an example POSIX TZ string for New Zealand after 2007. 649 It says that standard time (NZST) is 12 hours ahead of UTC, 650 and that daylight saving time (NZDT) is observed from September's 651 last Sunday at 02:00 until April's first Sunday at 03:00: 652 653 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> 654 655 This POSIX TZ string is hard to remember, and mishandles some 656 timestamps before 2008. With this package you can use this 657 instead: 658 659 <pre><code>TZ='Pacific/Auckland'</code></pre> 660 </li> 661 <li> 662 POSIX does not define the exact meaning of TZ values like 663 "<code>EST5EDT</code>". 664 Typically the current US DST rules are used to interpret such values, 665 but this means that the US DST rules are compiled into each program 666 that does time conversion. This means that when US time conversion 667 rules change (as in the United States in 1987), all programs that 668 do time conversion must be recompiled to ensure proper results. 669 </li> 670 <li> 671 The TZ environment variable is process-global, which makes it hard 672 to write efficient, thread-safe applications that need access 673 to multiple time zones. 674 </li> 675 <li> 676 In POSIX, there's no tamper-proof way for a process to learn the 677 system's best idea of local wall clock. (This is important for 678 applications that an administrator wants used only at certain 679 times – 680 without regard to whether the user has fiddled the TZ environment 681 variable. While an administrator can "do everything in UTC" to get 682 around the problem, doing so is inconvenient and precludes handling 683 daylight saving time shifts - as might be required to limit phone 684 calls to off-peak hours.) 685 </li> 686 <li> 687 POSIX provides no convenient and efficient way to determine the UT 688 offset and time zone abbreviation of arbitrary timestamps, 689 particularly for time zone settings that do not fit into the 690 POSIX model. 691 </li> 692 <li> 693 POSIX requires that systems ignore leap seconds. 694 </li> 695 <li> 696 The tz code attempts to support all the <code>time_t</code> 697 implementations allowed by POSIX. The <code>time_t</code> 698 type represents a nonnegative count of 699 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 700 In practice, <code>time_t</code> is usually a signed 64- or 701 32-bit integer; 32-bit signed <code>time_t</code> values stop 702 working after 2038-01-19 03:14:07 UTC, so 703 new implementations these days typically use a signed 64-bit integer. 704 Unsigned 32-bit integers are used on one or two platforms, 705 and 36-bit and 40-bit integers are also used occasionally. 706 Although earlier POSIX versions allowed <code>time_t</code> to be a 707 floating-point type, this was not supported by any practical 708 systems, and POSIX.1-2013 and the tz code both 709 require <code>time_t</code> 710 to be an integer type. 711 </li> 712 </ul> 713 <p> 714 These are the extensions that have been made to the POSIX functions: 715 </p> 716 <ul> 717 <li> 718 <p> 719 The TZ environment variable is used in generating the name of a file 720 from which time zone information is read (or is interpreted a la 721 POSIX); TZ is no longer constrained to be a three-letter time zone 722 name followed by a number of hours and an optional three-letter 723 daylight time zone name. The daylight saving time rules to be used 724 for a particular time zone are encoded in the time zone file; 725 the format of the file allows U.S., Australian, and other rules to be 726 encoded, and allows for situations where more than two time zone 727 abbreviations are used. 728 </p> 729 <p> 730 It was recognized that allowing the TZ environment variable to 731 take on values such as '<code>America/New_York</code>' might 732 cause "old" programs 733 (that expect TZ to have a certain form) to operate incorrectly; 734 consideration was given to using some other environment variable 735 (for example, TIMEZONE) to hold the string used to generate the 736 time zone information file name. In the end, however, it was decided 737 to continue using TZ: it is widely used for time zone purposes; 738 separately maintaining both TZ and TIMEZONE seemed a nuisance; 739 and systems where "new" forms of TZ might cause problems can simply 740 use TZ values such as "<code>EST5EDT</code>" which can be used both by 741 "new" programs (a la POSIX) and "old" programs (as zone names and 742 offsets). 743 </p> 744 </li> 745 <li> 746 The code supports platforms with a UT offset member 747 in <code>struct tm</code>, 748 e.g., <code>tm_gmtoff</code>. 749 </li> 750 <li> 751 The code supports platforms with a time zone abbreviation member in 752 <code>struct tm</code>, e.g., <code>tm_zone</code>. 753 </li> 754 <li> 755 Since the TZ environment variable can now be used to control time 756 conversion, the <code>daylight</code> 757 and <code>timezone</code> variables are no longer needed. 758 (These variables are defined and set by <code>tzset</code>; 759 however, their values will not be used 760 by <code>localtime</code>.) 761 </li> 762 <li> 763 Functions <code>tzalloc</code>, <code>tzfree</code>, 764 <code>localtime_rz</code>, and <code>mktime_z</code> for 765 more-efficient thread-safe applications that need to use 766 multiple time zones. The <code>tzalloc</code> 767 and <code>tzfree</code> functions allocate and free objects of 768 type <code>timezone_t</code>, and <code>localtime_rz</code> 769 and <code>mktime_z</code> are like <code>localtime_r</code> 770 and <code>mktime</code> with an extra 771 <code>timezone_t</code> argument. The functions were inspired 772 by NetBSD. 773 </li> 774 <li> 775 A function <code>tzsetwall</code> has been added to arrange 776 for the system's 777 best approximation to local wall clock time to be delivered by 778 subsequent calls to <code>localtime</code>. Source code for portable 779 applications that "must" run on local wall clock time should call 780 <code>tzsetwall</code>; if such code is moved to "old" systems that don't 781 provide tzsetwall, you won't be able to generate an executable program. 782 (These time zone functions also arrange for local wall clock time to be 783 used if tzset is called – directly or indirectly – 784 and there's no TZ 785 environment variable; portable applications should not, however, rely 786 on this behavior since it's not the way SVR2 systems behave.) 787 </li> 788 <li> 789 Negative <code>time_t</code> values are supported, on systems 790 where <code>time_t</code> is signed. 791 </li> 792 <li> 793 These functions can account for leap seconds, thanks to Bradley White. 794 </li> 795 </ul> 796 <p> 797 Points of interest to folks with other systems: 798 </p> 799 <ul> 800 <li> 801 Code compatible with this package is already part of many platforms, 802 including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS, 803 BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris. 804 On such hosts, the primary use of this package 805 is to update obsolete time zone rule tables. 806 To do this, you may need to compile the time zone compiler 807 '<code>zic</code>' supplied with this package instead of using 808 the system '<code>zic</code>', since the format 809 of <code>zic</code>'s input is occasionally extended, and a 810 platform may still be shipping an older <code>zic</code>. 811 </li> 812 <li> 813 The UNIX Version 7 <code>timezone</code> function is not 814 present in this package; 815 it's impossible to reliably map timezone's arguments (a "minutes west 816 of GMT" value and a "daylight saving time in effect" flag) to a 817 time zone abbreviation, and we refuse to guess. 818 Programs that in the past used the timezone function may now examine 819 <code>localtime(&clock)->tm_zone</code> 820 (if <code>TM_ZONE</code> is defined) or 821 <code>tzname[localtime(&clock)->tm_isdst]</code> 822 (if <code>HAVE_TZNAME</code> is defined) 823 to learn the correct time zone abbreviation to use. 824 </li> 825 <li> 826 The 4.2BSD <code>gettimeofday</code> function is not used in 827 this package. 828 This formerly let users obtain the current UTC offset and DST flag, 829 but this functionality was removed in later versions of BSD. 830 </li> 831 <li> 832 In SVR2, time conversion fails for near-minimum or near-maximum 833 <code>time_t</code> values when doing conversions for places 834 that don't use UT. 835 This package takes care to do these conversions correctly. 836 A comment in the source code tells how to get compatibly wrong 837 results. 838 </li> 839 </ul> 840 <p> 841 The functions that are conditionally compiled 842 if <code>STD_INSPIRED</code> is defined 843 should, at this point, be looked on primarily as food for thought. They are 844 not in any sense "standard compatible" – some are not, in fact, 845 specified in <em>any</em> standard. They do, however, represent responses of 846 various authors to 847 standardization proposals. 848 </p> 849 850 <p> 851 Other time conversion proposals, in particular the one developed by folks at 852 Hewlett Packard, offer a wider selection of functions that provide capabilities 853 beyond those provided here. The absence of such functions from this package 854 is not meant to discourage the development, standardization, or use of such 855 functions. Rather, their absence reflects the decision to make this package 856 contain valid extensions to POSIX, to ensure its broad acceptability. If 857 more powerful time conversion functions can be standardized, so much the 858 better. 859 </p> 860 </section> 861 862 863 <section> 864 <h2 id="stability">Interface stability</h2> 865 <p> 866 The tz code and data supply the following interfaces: 867 </p> 868 <ul> 869 <li> 870 A set of zone names as per "<a href="#naming">Names of time zone 871 rules</a>" above. 872 </li> 873 <li> 874 Library functions described in "<a href="#functions">Time and date 875 functions</a>" above. 876 </li> 877 <li> 878 The programs <code>tzselect</code>, <code>zdump</code>, 879 and <code>zic</code>, documented in their man pages. 880 </li> 881 <li> 882 The format of <code>zic</code> input files, documented in 883 the <code>zic</code> man page. 884 </li> 885 <li> 886 The format of <code>zic</code> output files, documented in 887 the <code>tzfile</code> man page. 888 </li> 889 <li> 890 The format of zone table files, documented in <code>zone1970.tab</code>. 891 </li> 892 <li> 893 The format of the country code file, documented in <code>iso3166.tab</code>. 894 </li> 895 <li> 896 The version number of the code and data, as the first line of 897 the text file '<code>version</code>' in each release. 898 </li> 899 </ul> 900 <p> 901 Interface changes in a release attempt to preserve compatibility with 902 recent releases. For example, tz data files typically do not rely on 903 recently-added <code>zic</code> features, so that users can run 904 older <code>zic</code> versions to process newer data 905 files. <a href="tz-link.htm">Sources for time zone and daylight 906 saving time data</a> describes how 907 releases are tagged and distributed. 908 </p> 909 910 <p> 911 Interfaces not listed above are less stable. For example, users 912 should not rely on particular UT offsets or abbreviations for 913 timestamps, as data entries are often based on guesswork and these 914 guesses may be corrected or improved. 915 </p> 916 </section> 917 918 919 <section> 920 <h2 id="calendar">Calendrical issues</h2> 921 <p> 922 Calendrical issues are a bit out of scope for a time zone database, 923 but they indicate the sort of problems that we would run into if we 924 extended the time zone database further into the past. An excellent 925 resource in this area is Nachum Dershowitz and Edward M. Reingold, 926 <cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical 927 Calculations: Third Edition</a></cite>, Cambridge University Press (2008). 928 Other information and sources are given in the file '<samp>calendars</samp>' 929 in the tz distribution. They sometimes disagree. 930 </p> 931 </section> 932 933 934 <section> 935 <h2 id="planets">Time and time zones on other planets</h2> 936 <p> 937 Some people's work schedules use Mars time. Jet Propulsion Laboratory 938 (JPL) coordinators have kept Mars time on and off at least since 1997 939 for the Mars Pathfinder mission. Some of their family members have 940 also adapted to Mars time. Dozens of special Mars watches were built 941 for JPL workers who kept Mars time during the Mars Exploration 942 Rovers mission (2004). These timepieces look like normal Seikos and 943 Citizens but use Mars seconds rather than terrestrial seconds. 944 </p> 945 946 <p> 947 A Mars solar day is called a "sol" and has a mean period equal to 948 about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 949 divided into a conventional 24-hour clock, so each Mars second equals 950 about 1.02749125 terrestrial seconds. 951 </p> 952 953 <p> 954 The prime meridian of Mars goes through the center of the crater 955 Airy-0, named in honor of the British astronomer who built the 956 Greenwich telescope that defines Earth's prime meridian. Mean solar 957 time on the Mars prime meridian is called Mars Coordinated Time (MTC). 958 </p> 959 960 <p> 961 Each landed mission on Mars has adopted a different reference for 962 solar time keeping, so there is no real standard for Mars time zones. 963 For example, the Mars Exploration Rover project (2004) defined two 964 time zones "Local Solar Time A" and "Local Solar Time B" for its two 965 missions, each zone designed so that its time equals local true solar 966 time at approximately the middle of the nominal mission. Such a "time 967 zone" is not particularly suited for any application other than the 968 mission itself. 969 </p> 970 971 <p> 972 Many calendars have been proposed for Mars, but none have achieved 973 wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 974 sequential count of Mars solar days elapsed since about 1873-12-29 975 12:00 GMT. 976 </p> 977 978 <p> 979 In our solar system, Mars is the planet with time and calendar most 980 like Earth's. On other planets, Sun-based time and calendars would 981 work quite differently. For example, although Mercury's sidereal 982 rotation period is 58.646 Earth days, Mercury revolves around the Sun 983 so rapidly that an observer on Mercury's equator would see a sunrise 984 only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury 985 day. Venus is more complicated, partly because its rotation is 986 slightly retrograde: its year is 1.92 of its days. Gas giants like 987 Jupiter are trickier still, as their polar and equatorial regions 988 rotate at different rates, so that the length of a day depends on 989 latitude. This effect is most pronounced on Neptune, where the day is 990 about 12 hours at the poles and 18 hours at the equator. 991 </p> 992 993 <p> 994 Although the tz database does not support time on other planets, it is 995 documented here in the hopes that support will be added eventually. 996 </p> 997 998 <p> 999 Sources: 1000 </p> 1001 <ul> 1002 <li> 1003 Michael Allison and Robert Schmunk, 1004 "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical 1005 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" 1006 (2012-08-08). 1007 </li> 1008 <li> 1009 Jia-Rui Chong, 1010 "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays 1011 Fit for a Martian</a>", Los Angeles Times 1012 (2004-01-14), pp A1, A20-A21. 1013 </li> 1014 <li> 1015 Tom Chmielewski, 1016 "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet 1017 Lag Is Worse on Mars</a>", The Atlantic (2015-02-26) 1018 </li> 1019 <li> 1020 Matt Williams, 1021 "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How 1022 long is a day on the other planets of the solar system?</a>" 1023 (2017-04-27). 1024 </li> 1025 </ul> 1026 </section> 1027 1028 <footer> 1029 <hr> 1030 This file is in the public domain, so clarified as of 2009-05-17 by 1031 Arthur David Olson. 1032 </footer> 1033 </body> 1034 </html> 1035
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