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1 1.1 christos <!DOCTYPE html> 2 1.1 christos <html lang="en"> 3 1.1 christos <head> 4 1.1 christos <title>Theory and pragmatics of the tz code and data</title> 5 1.1 christos <meta charset="UTF-8"> 6 1.1 christos </head> 7 1.1 christos 8 1.1 christos <!-- The somewhat-unusal indenting style in this file is intended to 9 1.1 christos shrink the output of the shell command 'diff Theory Theory.html', 10 1.1 christos where 'Theory' was the plain text file that this file is derived 11 1.1 christos from. The 'Theory' file used leading white space to indent, and 12 1.1 christos when possible that indentation is preserved here. Eventually we 13 1.1 christos may stop doing this and remove this comment. --> 14 1.1 christos 15 1.1 christos <body> 16 1.1 christos <h1>Theory and pragmatics of the tz code and data</h1> 17 1.1 christos <h3>Outline</h3> 18 1.1 christos <nav> 19 1.1 christos <ul> 20 1.1 christos <li><a href="#scope">Scope of the tz database</a></li> 21 1.1 christos <li><a href="#naming">Names of time zone rules</a></li> 22 1.1 christos <li><a href="#abbreviations">Time zone abbreviations</a></li> 23 1.1 christos <li><a href="#accuracy">Accuracy of the tz database</a></li> 24 1.1 christos <li><a href="#functions">Time and date functions</a></li> 25 1.1 christos <li><a href="#stability">Interface stability</a></li> 26 1.1 christos <li><a href="#calendar">Calendrical issues</a></li> 27 1.1 christos <li><a href="#planets">Time and time zones on other planets</a></li> 28 1.1 christos </ul> 29 1.1 christos </nav> 30 1.1 christos 31 1.1 christos 32 1.1 christos <section> 33 1.1 christos <h2 id="scope">Scope of the tz database</h2> 34 1.1 christos <p> 35 1.1 christos The tz database attempts to record the history and predicted future of 36 1.1 christos all computer-based clocks that track civil time. To represent this 37 1.1 christos data, the world is partitioned into regions whose clocks all agree 38 1.1 christos about timestamps that occur after the somewhat-arbitrary cutoff point 39 1.1 christos of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, 40 1.1 christos the database records all known clock transitions, and labels the region 41 1.1 christos with a notable location. Although 1970 is a somewhat-arbitrary 42 1.1 christos cutoff, there are significant challenges to moving the cutoff earlier 43 1.1 christos even by a decade or two, due to the wide variety of local practices 44 1.1 christos before computer timekeeping became prevalent. 45 1.1 christos </p> 46 1.1 christos 47 1.1 christos <p> 48 1.1 christos Clock transitions before 1970 are recorded for each such location, 49 1.1 christos because most systems support timestamps before 1970 and could 50 1.1 christos misbehave if data entries were omitted for pre-1970 transitions. 51 1.1 christos However, the database is not designed for and does not suffice for 52 1.1 christos applications requiring accurate handling of all past times everywhere, 53 1.1 christos as it would take far too much effort and guesswork to record all 54 1.1 christos details of pre-1970 civil timekeeping. 55 1.1 christos </p> 56 1.1 christos 57 1.1 christos <p> 58 1.1 christos As described below, reference source code for using the tz database is 59 1.1 christos also available. The tz code is upwards compatible with POSIX, an 60 1.1 christos international standard for UNIX-like systems. As of this writing, the 61 1.1 christos current edition of POSIX is: 62 1.1 christos <a href="http://pubs.opengroup.org/onlinepubs/9699919799/"> 63 1.1 christos The Open Group Base Specifications Issue 7</a>, 64 1.1 christos IEEE Std 1003.1-2008, 2016 Edition. 65 1.1 christos </p> 66 1.1 christos </section> 67 1.1 christos 68 1.1 christos 69 1.1 christos 70 1.1 christos <section> 71 1.1 christos <h2 id="naming">Names of time zone rules</h2> 72 1.1 christos <p> 73 1.1 christos Each of the database's time zone rules has a unique name. 74 1.1 christos Inexperienced users are not expected to select these names unaided. 75 1.1 christos Distributors should provide documentation and/or a simple selection 76 1.1 christos interface that explains the names; for one example, see the 'tzselect' 77 1.1 christos program in the tz code. The 78 1.1 christos <a href="http://cldr.unicode.org/">Unicode Common Locale Data 79 1.1 christos Repository</a> contains data that may be useful for other 80 1.1 christos selection interfaces. 81 1.1 christos </p> 82 1.1 christos 83 1.1 christos <p> 84 1.1 christos The time zone rule naming conventions attempt to strike a balance 85 1.1 christos among the following goals: 86 1.1 christos </p> 87 1.1 christos <ul> 88 1.1 christos <li> 89 1.1 christos Uniquely identify every region where clocks have agreed since 1970. 90 1.1 christos This is essential for the intended use: static clocks keeping local 91 1.1 christos civil time. 92 1.1 christos </li> 93 1.1 christos <li> 94 1.1 christos Indicate to experts where that region is. 95 1.1 christos </li> 96 1.1 christos <li> 97 1.1 christos Be robust in the presence of political changes. For example, names 98 1.1 christos of countries are ordinarily not used, to avoid incompatibilities 99 1.1 christos when countries change their name (e.g. Zaire→Congo) or when 100 1.1 christos locations change countries (e.g. Hong Kong from UK colony to 101 1.1 christos China). 102 1.1 christos </li> 103 1.1 christos <li> 104 1.1 christos Be portable to a wide variety of implementations. 105 1.1 christos </li> 106 1.1 christos <li> 107 1.1 christos Use a consistent naming conventions over the entire world. 108 1.1 christos </li> 109 1.1 christos </ul> 110 1.1 christos <p> 111 1.1 christos Names normally have the 112 1.1 christos form <var>AREA</var><code>/</code><var>LOCATION</var>, 113 1.1 christos where <var>AREA</var> is the name of a continent or ocean, 114 1.1 christos and <var>LOCATION</var> is the name of a specific 115 1.1 christos location within that region. North and South America share the same 116 1.1 christos area, '<code>America</code>'. Typical names are 117 1.1 christos '<code>Africa/Cairo</code>', '<code>America/New_York</code>', and 118 1.1 christos '<code>Pacific/Honolulu</code>'. 119 1.1 christos </p> 120 1.1 christos 121 1.1 christos <p> 122 1.1 christos Here are the general rules used for choosing location names, 123 1.1 christos in decreasing order of importance: 124 1.1 christos </p> 125 1.1 christos <ul> 126 1.1 christos <li> 127 1.1 christos Use only valid POSIX file name components (i.e., the parts of 128 1.1 christos names other than '<code>/</code>'). Do not use the file name 129 1.1 christos components '<code>.</code>' and '<code>..</code>'. 130 1.1 christos Within a file name component, 131 1.1 christos use only ASCII letters, '<code>.</code>', 132 1.1 christos '<code>-</code>' and '<code>_</code>'. Do not use 133 1.1 christos digits, as that might create an ambiguity with POSIX 134 1.1 christos TZ strings. A file name component must not exceed 14 135 1.1 christos characters or start with '<code>-</code>'. E.g., 136 1.1 christos prefer '<code>Brunei</code>' to 137 1.1 christos '<code>Bandar_Seri_Begawan</code>'. Exceptions: see 138 1.1 christos the discussion 139 1.1 christos of legacy names below. 140 1.1 christos </li> 141 1.1 christos <li> 142 1.1 christos A name must not be empty, or contain '<code>//</code>', or 143 1.1 christos start or end with '<code>/</code>'. 144 1.1 christos </li> 145 1.1 christos <li> 146 1.1 christos Do not use names that differ only in case. Although the reference 147 1.1 christos implementation is case-sensitive, some other implementations 148 1.1 christos are not, and they would mishandle names differing only in case. 149 1.1 christos </li> 150 1.1 christos <li> 151 1.1 christos If one name <var>A</var> is an initial prefix of another 152 1.1 christos name <var>AB</var> (ignoring case), then <var>B</var> 153 1.1 christos must not start with '<code>/</code>', as a 154 1.1 christos regular file cannot have 155 1.1 christos the same name as a directory in POSIX. For example, 156 1.1 christos '<code>America/New_York</code>' precludes 157 1.1 christos '<code>America/New_York/Bronx</code>'. 158 1.1 christos </li> 159 1.1 christos <li> 160 1.1 christos Uninhabited regions like the North Pole and Bouvet Island 161 1.1 christos do not need locations, since local time is not defined there. 162 1.1 christos </li> 163 1.1 christos <li> 164 1.1 christos There should typically be at least one name for each ISO 3166-1 165 1.1 christos officially assigned two-letter code for an inhabited country 166 1.1 christos or territory. 167 1.1 christos </li> 168 1.1 christos <li> 169 1.1 christos If all the clocks in a region have agreed since 1970, 170 1.1 christos don't bother to include more than one location 171 1.1 christos even if subregions' clocks disagreed before 1970. 172 1.1 christos Otherwise these tables would become annoyingly large. 173 1.1 christos </li> 174 1.1 christos <li> 175 1.1 christos If a name is ambiguous, use a less ambiguous alternative; 176 1.1 christos e.g. many cities are named San Jos and Georgetown, so 177 1.1 christos prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'. 178 1.1 christos </li> 179 1.1 christos <li> 180 1.1 christos Keep locations compact. Use cities or small islands, not countries 181 1.1 christos or regions, so that any future time zone changes do not split 182 1.1 christos locations into different time zones. E.g. prefer 183 1.1 christos '<code>Paris</code>' to '<code>France</code>', since 184 1.1 christos France has had multiple time zones. 185 1.1 christos </li> 186 1.1 christos <li> 187 1.1 christos Use mainstream English spelling, e.g. prefer 188 1.1 christos '<code>Rome</code>' to '<code>Roma</code>', and prefer 189 1.1 christos '<code>Athens</code>' to the Greek 190 1.1 christos '<code></code>' or the Romanized 191 1.1 christos '<code>Athna</code>'. 192 1.1 christos The POSIX file name restrictions encourage this rule. 193 1.1 christos </li> 194 1.1 christos <li> 195 1.1 christos Use the most populous among locations in a zone, 196 1.1 christos e.g. prefer '<code>Shanghai</code>' to 197 1.1 christos '<code>Beijing</code>'. Among locations with 198 1.1 christos similar populations, pick the best-known location, 199 1.1 christos e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'. 200 1.1 christos </li> 201 1.1 christos <li> 202 1.1 christos Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'. 203 1.1 christos </li> 204 1.1 christos <li> 205 1.1 christos Omit common suffixes like '<code>_Islands</code>' and 206 1.1 christos '<code>_City</code>', unless that would lead to 207 1.1 christos ambiguity. E.g. prefer '<code>Cayman</code>' to 208 1.1 christos '<code>Cayman_Islands</code>' and 209 1.1 christos '<code>Guatemala</code>' to 210 1.1 christos '<code>Guatemala_City</code>', but prefer 211 1.1 christos '<code>Mexico_City</code>' to '<code>Mexico</code>' 212 1.1 christos because the country 213 1.1 christos of Mexico has several time zones. 214 1.1 christos </li> 215 1.1 christos <li> 216 1.1 christos Use '<code>_</code>' to represent a space. 217 1.1 christos </li> 218 1.1 christos <li> 219 1.1 christos Omit '<code>.</code>' from abbreviations in names, e.g. prefer 220 1.1 christos '<code>St_Helena</code>' to '<code>St._Helena</code>'. 221 1.1 christos </li> 222 1.1 christos <li> 223 1.1 christos Do not change established names if they only marginally 224 1.1 christos violate the above rules. For example, don't change 225 1.1 christos the existing name '<code>Rome</code>' to 226 1.1 christos '<code>Milan</code>' merely because 227 1.1 christos Milan's population has grown to be somewhat greater 228 1.1 christos than Rome's. 229 1.1 christos </li> 230 1.1 christos <li> 231 1.1 christos If a name is changed, put its old spelling in the 232 1.1 christos '<code>backward</code>' file. 233 1.1 christos This means old spellings will continue to work. 234 1.1 christos </li> 235 1.1 christos </ul> 236 1.1 christos 237 1.1 christos <p> 238 1.1 christos The file '<code>zone1970.tab</code>' lists geographical locations used 239 1.1 christos to name time 240 1.1 christos zone rules. It is intended to be an exhaustive list of names for 241 1.1 christos geographic regions as described above; this is a subset of the names 242 1.1 christos in the data. Although a '<code>zone1970.tab</code>' location's longitude 243 1.1 christos corresponds to its LMT offset with one hour for every 15 degrees east 244 1.1 christos longitude, this relationship is not exact. 245 1.1 christos </p> 246 1.1 christos 247 1.1 christos <p> 248 1.1 christos Older versions of this package used a different naming scheme, 249 1.1 christos and these older names are still supported. 250 1.1 christos See the file '<code>backward</code>' for most of these older names 251 1.1 christos (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). 252 1.1 christos The other old-fashioned names still supported are 253 1.1 christos '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>'). 254 1.1 christos </p> 255 1.1 christos 256 1.1 christos <p> 257 1.1 christos Older versions of this package defined legacy names that are 258 1.1 christos incompatible with the first rule of location names, but which are 259 1.1 christos still supported. These legacy names are mostly defined in the file 260 1.1 christos '<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names 261 1.1 christos '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the 262 1.1 christos legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'. 263 1.1 christos </p> 264 1.1 christos 265 1.1 christos <p> 266 1.1 christos Excluding '<code>backward</code>' should not affect the other data. If 267 1.1 christos '<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the 268 1.1 christos remaining data. 269 1.1 christos </p> 270 1.1 christos 271 1.1 christos 272 1.1 christos </section> 273 1.1 christos <section> 274 1.1 christos <h2 id="abbreviations">Time zone abbreviations</h2> 275 1.1 christos <p> 276 1.1 christos When this package is installed, it generates time zone abbreviations 277 1.1 christos like '<code>EST</code>' to be compatible with human tradition and POSIX. 278 1.1 christos Here are the general rules used for choosing time zone abbreviations, 279 1.1 christos in decreasing order of importance: 280 1.1 christos <ul> 281 1.1 christos <li> 282 1.1 christos Use three or more characters that are ASCII alphanumerics or 283 1.1 christos '<code>+</code>' or '<code>-</code>'. 284 1.1 christos Previous editions of this database also used characters like 285 1.1 christos '<code> </code>' and '<code>?</code>', but these 286 1.1 christos characters have a special meaning to 287 1.1 christos the shell and cause commands like 288 1.1 christos '<code>set `date`</code>' 289 1.1 christos to have unexpected effects. 290 1.1 christos Previous editions of this rule required upper-case letters, 291 1.1 christos but the Congressman who introduced Chamorro Standard Time 292 1.1 christos preferred "ChST", so lower-case letters are now allowed. 293 1.1 christos Also, POSIX from 2001 on relaxed the rule to allow 294 1.1 christos '<code>-</code>', '<code>+</code>', 295 1.1 christos and alphanumeric characters from the portable character set 296 1.1 christos in the current locale. In practice ASCII alphanumerics and 297 1.1 christos '<code>+</code>' and '<code>-</code>' are safe in all locales. 298 1.1 christos 299 1.1 christos In other words, in the C locale the POSIX extended regular 300 1.1 christos expression <code>[-+[:alnum:]]{3,}</code> should match 301 1.1 christos the abbreviation. 302 1.1 christos This guarantees that all abbreviations could have been 303 1.1 christos specified by a POSIX TZ string. 304 1.1 christos </li> 305 1.1 christos <li> 306 1.1 christos Use abbreviations that are in common use among English-speakers, 307 1.1 christos e.g. 'EST' for Eastern Standard Time in North America. 308 1.1 christos We assume that applications translate them to other languages 309 1.1 christos as part of the normal localization process; for example, 310 1.1 christos a French application might translate 'EST' to 'HNE'. 311 1.1 christos </li> 312 1.1 christos <li> 313 1.1 christos For zones whose times are taken from a city's longitude, use the 314 1.1 christos traditional <var>x</var>MT notation, e.g. 'PMT' for 315 1.1 christos Paris Mean Time. 316 1.1 christos The only name like this in current use is 'GMT'. 317 1.1 christos </li> 318 1.1 christos <li> 319 1.1 christos Use 'LMT' for local mean time of locations before the introduction 320 1.1 christos of standard time; see "<a href="#scope">Scope of the 321 1.1 christos tz database</a>". 322 1.1 christos </li> 323 1.1 christos <li> 324 1.1 christos If there is no common English abbreviation, use numeric offsets like 325 1.1 christos <code>-</code>05 and <code>+</code>0830 that are 326 1.1 christos generated by zic's <code>%z</code> notation. 327 1.1 christos </li> 328 1.1 christos <li> 329 1.1 christos Use current abbreviations for older timestamps to avoid confusion. 330 1.1 christos For example, in 1910 a common English abbreviation for UT +01 331 1.1 christos in central Europe was 'MEZ' (short for both "Middle European 332 1.1 christos Zone" and for "Mitteleuropische Zeit" in German). Nowadays 333 1.1 christos 'CET' ("Central European Time") is more common in English, and 334 1.1 christos the database uses 'CET' even for circa-1910 timestamps as this 335 1.1 christos is less confusing for modern users and avoids the need for 336 1.1 christos determining when 'CET' supplanted 'MEZ' in common usage. 337 1.1 christos </li> 338 1.1 christos <li> 339 1.1 christos Use a consistent style in a zone's history. For example, if a zone's 340 1.1 christos history tends to use numeric abbreviations and a particular 341 1.1 christos entry could go either way, use a numeric abbreviation. 342 1.1 christos </li> 343 1.1 christos </ul> 344 1.1 christos [The remaining guidelines predate the introduction of <code>%z</code>. 345 1.1 christos They are problematic as they mean tz data entries invent 346 1.1 christos notation rather than record it. These guidelines are now 347 1.1 christos deprecated and the plan is to gradually move to <code>%z</code> for 348 1.1 christos inhabited locations and to "<code>-</code>00" for uninhabited locations.] 349 1.1 christos <ul> 350 1.1 christos <li> 351 1.1 christos If there is no common English abbreviation, abbreviate the English 352 1.1 christos translation of the usual phrase used by native speakers. 353 1.1 christos If this is not available or is a phrase mentioning the country 354 1.1 christos (e.g. "Cape Verde Time"), then: 355 1.1 christos <ul> 356 1.1 christos <li> 357 1.1 christos When a country is identified with a single or principal zone, 358 1.1 christos append 'T' to the country's ISO code, e.g. 'CVT' for 359 1.1 christos Cape Verde Time. For summer time append 'ST'; 360 1.1 christos for double summer time append 'DST'; etc. 361 1.1 christos </li> 362 1.1 christos <li> 363 1.1 christos Otherwise, take the first three letters of an English place 364 1.1 christos name identifying each zone and append 'T', 'ST', etc. 365 1.1 christos as before; e.g. 'CHAST' for CHAtham Summer Time. 366 1.1 christos </li> 367 1.1 christos </ul> 368 1.1 christos </li> 369 1.1 christos <li> 370 1.1 christos Use UT (with time zone abbreviation '<code>-</code>00') for 371 1.1 christos locations while uninhabited. The leading 372 1.1 christos '<code>-</code>' is a flag that the time 373 1.1 christos zone is in some sense undefined; this notation is 374 1.1 christos derived from Internet RFC 3339. 375 1.1 christos </li> 376 1.1 christos </ul> 377 1.1 christos <p> 378 1.1 christos Application writers should note that these abbreviations are ambiguous 379 1.1 christos in practice: e.g. 'CST' has a different meaning in China than 380 1.1 christos it does in the United States. In new applications, it's often better 381 1.1 christos to use numeric UT offsets like '<code>-</code>0600' instead of time zone 382 1.1 christos abbreviations like 'CST'; this avoids the ambiguity. 383 1.1 christos </p> 384 1.1 christos </section> 385 1.1 christos 386 1.1 christos 387 1.1 christos <section> 388 1.1 christos <h2 id="accuracy">Accuracy of the tz database</h2> 389 1.1 christos <p> 390 1.1 christos The tz database is not authoritative, and it surely has errors. 391 1.1 christos Corrections are welcome and encouraged; see the file CONTRIBUTING. 392 1.1 christos Users requiring authoritative data should consult national standards 393 1.1 christos bodies and the references cited in the database's comments. 394 1.1 christos </p> 395 1.1 christos 396 1.1 christos <p> 397 1.1 christos Errors in the tz database arise from many sources: 398 1.1 christos </p> 399 1.1 christos <ul> 400 1.1 christos <li> 401 1.1 christos The tz database predicts future timestamps, and current predictions 402 1.1 christos will be incorrect after future governments change the rules. 403 1.1 christos For example, if today someone schedules a meeting for 13:00 next 404 1.1 christos October 1, Casablanca time, and tomorrow Morocco changes its 405 1.1 christos daylight saving rules, software can mess up after the rule change 406 1.1 christos if it blithely relies on conversions made before the change. 407 1.1 christos </li> 408 1.1 christos <li> 409 1.1 christos The pre-1970 entries in this database cover only a tiny sliver of how 410 1.1 christos clocks actually behaved; the vast majority of the necessary 411 1.1 christos information was lost or never recorded. Thousands more zones would 412 1.1 christos be needed if the tz database's scope were extended to cover even 413 1.1 christos just the known or guessed history of standard time; for example, 414 1.1 christos the current single entry for France would need to split into dozens 415 1.1 christos of entries, perhaps hundreds. And in most of the world even this 416 1.1 christos approach would be misleading due to widespread disagreement or 417 1.1 christos indifference about what times should be observed. In her 2015 book 418 1.1 christos <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes 419 1.1 christos "Outside of Europe and North America there was no system of time 420 1.1 christos zones at all, often not even a stable landscape of mean times, 421 1.1 christos prior to the middle decades of the twentieth century". See: 422 1.1 christos Timothy Shenk, <a 423 1.1 christos href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: 424 1.1 christos A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. 425 1.1 christos </li> 426 1.1 christos <li> 427 1.1 christos Most of the pre-1970 data entries come from unreliable sources, often 428 1.1 christos astrology books that lack citations and whose compilers evidently 429 1.1 christos invented entries when the true facts were unknown, without 430 1.1 christos reporting which entries were known and which were invented. 431 1.1 christos These books often contradict each other or give implausible entries, 432 1.1 christos and on the rare occasions when they are checked they are 433 1.1 christos typically found to be incorrect. 434 1.1 christos </li> 435 1.1 christos <li> 436 1.1 christos For the UK the tz database relies on years of first-class work done by 437 1.1 christos Joseph Myers and others; see 438 1.1 christos "<a href="https://www.polyomino.org.uk/british-time/">History of 439 1.1 christos legal time in Britain</a>". 440 1.1 christos Other countries are not done nearly as well. 441 1.1 christos </li> 442 1.1 christos <li> 443 1.1 christos Sometimes, different people in the same city would maintain clocks 444 1.1 christos that differed significantly. Railway time was used by railroad 445 1.1 christos companies (which did not always agree with each other), 446 1.1 christos church-clock time was used for birth certificates, etc. 447 1.1 christos Often this was merely common practice, but sometimes it was set by law. 448 1.1 christos For example, from 1891 to 1911 the UT offset in France was legally 449 1.1 christos 0:09:21 outside train stations and 0:04:21 inside. 450 1.1 christos </li> 451 1.1 christos <li> 452 1.1 christos Although a named location in the tz database stands for the 453 1.1 christos containing region, its pre-1970 data entries are often accurate for 454 1.1 christos only a small subset of that region. For example, <code>Europe/London</code> 455 1.1 christos stands for the United Kingdom, but its pre-1847 times are valid 456 1.1 christos only for locations that have London's exact meridian, and its 1847 457 1.1 christos transition to GMT is known to be valid only for the L&NW and the 458 1.1 christos Caledonian railways. 459 1.1 christos </li> 460 1.1 christos <li> 461 1.1 christos The tz database does not record the earliest time for which a zone's 462 1.1 christos data entries are thereafter valid for every location in the region. 463 1.1 christos For example, <code>Europe/London</code> is valid for all locations in its 464 1.1 christos region after GMT was made the standard time, but the date of 465 1.1 christos standardization (1880-08-02) is not in the tz database, other than 466 1.1 christos in commentary. For many zones the earliest time of validity is 467 1.1 christos unknown. 468 1.1 christos </li> 469 1.1 christos <li> 470 1.1 christos The tz database does not record a region's boundaries, and in many 471 1.1 christos cases the boundaries are not known. For example, the zone 472 1.1 christos <code>America/Kentucky/Louisville</code> represents a region around 473 1.1 christos the city of 474 1.1 christos Louisville, the boundaries of which are unclear. 475 1.1 christos </li> 476 1.1 christos <li> 477 1.1 christos Changes that are modeled as instantaneous transitions in the tz 478 1.1 christos database were often spread out over hours, days, or even decades. 479 1.1 christos </li> 480 1.1 christos <li> 481 1.1 christos Even if the time is specified by law, locations sometimes 482 1.1 christos deliberately flout the law. 483 1.1 christos </li> 484 1.1 christos <li> 485 1.1 christos Early timekeeping practices, even assuming perfect clocks, were 486 1.1 christos often not specified to the accuracy that the tz database requires. 487 1.1 christos </li> 488 1.1 christos <li> 489 1.1 christos Sometimes historical timekeeping was specified more precisely 490 1.1 christos than what the tz database can handle. For example, from 1909 to 491 1.1 christos 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz 492 1.1 christos database cannot represent the fractional second. 493 1.1 christos </li> 494 1.1 christos <li> 495 1.1 christos Even when all the timestamp transitions recorded by the tz database 496 1.1 christos are correct, the tz rules that generate them may not faithfully 497 1.1 christos reflect the historical rules. For example, from 1922 until World 498 1.1 christos War II the UK moved clocks forward the day following the third 499 1.1 christos Saturday in April unless that was Easter, in which case it moved 500 1.1 christos clocks forward the previous Sunday. Because the tz database has no 501 1.1 christos way to specify Easter, these exceptional years are entered as 502 1.1 christos separate tz Rule lines, even though the legal rules did not change. 503 1.1 christos </li> 504 1.1 christos <li> 505 1.1 christos The tz database models pre-standard time using the proleptic Gregorian 506 1.1 christos calendar and local mean time (LMT), but many people used other 507 1.1 christos calendars and other timescales. For example, the Roman Empire used 508 1.1 christos the Julian calendar, and had 12 varying-length daytime hours with a 509 1.1 christos non-hour-based system at night. 510 1.1 christos </li> 511 1.1 christos <li> 512 1.1 christos Early clocks were less reliable, and data entries do not represent 513 1.1 christos clock error. 514 1.1 christos </li> 515 1.1 christos <li> 516 1.1 christos The tz database assumes Universal Time (UT) as an origin, even 517 1.1 christos though UT is not standardized for older timestamps. In the tz 518 1.1 christos database commentary, UT denotes a family of time standards that 519 1.1 christos includes Coordinated Universal Time (UTC) along with other variants 520 1.1 christos such as UT1 and GMT, with days starting at midnight. Although UT 521 1.1 christos equals UTC for modern timestamps, UTC was not defined until 1960, 522 1.1 christos so commentary uses the more-general abbreviation UT for timestamps 523 1.1 christos that might predate 1960. Since UT, UT1, etc. disagree slightly, 524 1.1 christos and since pre-1972 UTC seconds varied in length, interpretation of 525 1.1 christos older timestamps can be problematic when subsecond accuracy is 526 1.1 christos needed. 527 1.1 christos </li> 528 1.1 christos <li> 529 1.1 christos Civil time was not based on atomic time before 1972, and we don't 530 1.1 christos know the history of earth's rotation accurately enough to map SI 531 1.1 christos seconds to historical solar time to more than about one-hour 532 1.1 christos accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY. 533 1.1 christos <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement 534 1.1 christos of the Earth's rotation: 720 BC to AD 2015</a>. 535 1.1 christos <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. 536 1.1 christos Also see: Espenak F. <a 537 1.1 christos href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty 538 1.1 christos in Delta T (T)</a>. 539 1.1 christos </li> 540 1.1 christos <li> 541 1.1 christos The relationship between POSIX time (that is, UTC but ignoring leap 542 1.1 christos seconds) and UTC is not agreed upon after 1972. Although the POSIX 543 1.1 christos clock officially stops during an inserted leap second, at least one 544 1.1 christos proposed standard has it jumping back a second instead; and in 545 1.1 christos practice POSIX clocks more typically either progress glacially during 546 1.1 christos a leap second, or are slightly slowed while near a leap second. 547 1.1 christos </li> 548 1.1 christos <li> 549 1.1 christos The tz database does not represent how uncertain its information is. 550 1.1 christos Ideally it would contain information about when data entries are 551 1.1 christos incomplete or dicey. Partial temporal knowledge is a field of 552 1.1 christos active research, though, and it's not clear how to apply it here. 553 1.1 christos </li> 554 1.1 christos </ul> 555 1.1 christos <p> 556 1.1 christos In short, many, perhaps most, of the tz database's pre-1970 and future 557 1.1 christos timestamps are either wrong or misleading. Any attempt to pass the 558 1.1 christos tz database off as the definition of time should be unacceptable to 559 1.1 christos anybody who cares about the facts. In particular, the tz database's 560 1.1 christos LMT offsets should not be considered meaningful, and should not prompt 561 1.1 christos creation of zones merely because two locations differ in LMT or 562 1.1 christos transitioned to standard time at different dates. 563 1.1 christos </p> 564 1.1 christos </section> 565 1.1 christos 566 1.1 christos 567 1.1 christos <section> 568 1.1 christos <h2 id="functions">Time and date functions</h2> 569 1.1 christos <p> 570 1.1 christos The tz code contains time and date functions that are upwards 571 1.1 christos compatible with those of POSIX. 572 1.1 christos </p> 573 1.1 christos 574 1.1 christos <p> 575 1.1 christos POSIX has the following properties and limitations. 576 1.1 christos </p> 577 1.1 christos <ul> 578 1.1 christos <li> 579 1.1 christos <p> 580 1.1 christos In POSIX, time display in a process is controlled by the 581 1.1 christos environment variable TZ. Unfortunately, the POSIX TZ string takes 582 1.1 christos a form that is hard to describe and is error-prone in practice. 583 1.1 christos Also, POSIX TZ strings can't deal with other (for example, Israeli) 584 1.1 christos daylight saving time rules, or situations where more than two 585 1.1 christos time zone abbreviations are used in an area. 586 1.1 christos </p> 587 1.1 christos <p> 588 1.1 christos The POSIX TZ string takes the following form: 589 1.1 christos </p> 590 1.1 christos <p> 591 1.1 christos <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 1.1 christos </p> 593 1.1 christos <p> 594 1.1 christos where: 595 1.1 christos <dl> 596 1.1 christos <dt><var>std</var> and <var>dst</var></dt><dd> 597 1.1 christos are 3 or more characters specifying the standard 598 1.1 christos and daylight saving time (DST) zone names. 599 1.1 christos Starting with POSIX.1-2001, <var>std</var> 600 1.1 christos and <var>dst</var> may also be 601 1.1 christos in a quoted form like '<code><UTC+10></code>'; this allows 602 1.1 christos "<code>+</code>" and "<code>-</code>" in the names. 603 1.1 christos </dd> 604 1.1 christos <dt><var>offset</var></dt><dd> 605 1.1 christos is of the form 606 1.1 christos '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' 607 1.1 christos and specifies the offset west of UT. '<var>hh</var>' 608 1.1 christos may be a single digit; 0≤<var>hh</var>≤24. 609 1.1 christos The default DST offset is one hour ahead of standard time. 610 1.1 christos </dd> 611 1.1 christos <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> 612 1.1 christos specifies the beginning and end of DST. If this is absent, 613 1.1 christos the system supplies its own rules for DST, and these can 614 1.1 christos differ from year to year; typically US DST rules are used. 615 1.1 christos </dd> 616 1.1 christos <dt><var>time</var></dt><dd> 617 1.1 christos takes the form 618 1.1 christos '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' 619 1.1 christos and defaults to 02:00. 620 1.1 christos This is the same format as the offset, except that a 621 1.1 christos leading '<code>+</code>' or '<code>-</code>' is not allowed. 622 1.1 christos </dd> 623 1.1 christos <dt><var>date</var></dt><dd> 624 1.1 christos takes one of the following forms: 625 1.1 christos <dl> 626 1.1 christos <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> 627 1.1 christos origin-1 day number not counting February 29 628 1.1 christos </dd> 629 1.1 christos <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> 630 1.1 christos origin-0 day number counting February 29 if present 631 1.1 christos </dd> 632 1.1 christos <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 1.1 christos for the <var>d</var>th day of 634 1.1 christos week <var>n</var> of month <var>m</var> of the 635 1.1 christos year, where week 1 is the first week in which 636 1.1 christos day <var>d</var> appears, and '<code>5</code>' 637 1.1 christos stands for the last week in which 638 1.1 christos day <var>d</var> appears 639 1.1 christos (which may be either the 4th or 5th week). 640 1.1 christos Typically, this is the only useful form; 641 1.1 christos the <var>n</var> 642 1.1 christos and <code>J</code><var>n</var> forms are 643 1.1 christos rarely used. 644 1.1 christos </dd> 645 1.1 christos </dl> 646 1.1 christos </dd> 647 1.1 christos </dl> 648 1.1 christos Here is an example POSIX TZ string for New Zealand after 2007. 649 1.1 christos It says that standard time (NZST) is 12 hours ahead of UTC, 650 1.1 christos and that daylight saving time (NZDT) is observed from September's 651 1.1 christos last Sunday at 02:00 until April's first Sunday at 03:00: 652 1.1 christos 653 1.1 christos <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> 654 1.1 christos 655 1.1 christos This POSIX TZ string is hard to remember, and mishandles some 656 1.1 christos timestamps before 2008. With this package you can use this 657 1.1 christos instead: 658 1.1 christos 659 1.1 christos <pre><code>TZ='Pacific/Auckland'</code></pre> 660 1.1 christos </li> 661 1.1 christos <li> 662 1.1 christos POSIX does not define the exact meaning of TZ values like 663 1.1 christos "<code>EST5EDT</code>". 664 1.1 christos Typically the current US DST rules are used to interpret such values, 665 1.1 christos but this means that the US DST rules are compiled into each program 666 1.1 christos that does time conversion. This means that when US time conversion 667 1.1 christos rules change (as in the United States in 1987), all programs that 668 1.1 christos do time conversion must be recompiled to ensure proper results. 669 1.1 christos </li> 670 1.1 christos <li> 671 1.1 christos The TZ environment variable is process-global, which makes it hard 672 1.1 christos to write efficient, thread-safe applications that need access 673 1.1 christos to multiple time zones. 674 1.1 christos </li> 675 1.1 christos <li> 676 1.1 christos In POSIX, there's no tamper-proof way for a process to learn the 677 1.1 christos system's best idea of local wall clock. (This is important for 678 1.1 christos applications that an administrator wants used only at certain 679 1.1 christos times – 680 1.1 christos without regard to whether the user has fiddled the TZ environment 681 1.1 christos variable. While an administrator can "do everything in UTC" to get 682 1.1 christos around the problem, doing so is inconvenient and precludes handling 683 1.1 christos daylight saving time shifts - as might be required to limit phone 684 1.1 christos calls to off-peak hours.) 685 1.1 christos </li> 686 1.1 christos <li> 687 1.1 christos POSIX provides no convenient and efficient way to determine the UT 688 1.1 christos offset and time zone abbreviation of arbitrary timestamps, 689 1.1 christos particularly for time zone settings that do not fit into the 690 1.1 christos POSIX model. 691 1.1 christos </li> 692 1.1 christos <li> 693 1.1 christos POSIX requires that systems ignore leap seconds. 694 1.1 christos </li> 695 1.1 christos <li> 696 1.1 christos The tz code attempts to support all the <code>time_t</code> 697 1.1 christos implementations allowed by POSIX. The <code>time_t</code> 698 1.1 christos type represents a nonnegative count of 699 1.1 christos seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. 700 1.1 christos In practice, <code>time_t</code> is usually a signed 64- or 701 1.1 christos 32-bit integer; 32-bit signed <code>time_t</code> values stop 702 1.1 christos working after 2038-01-19 03:14:07 UTC, so 703 1.1 christos new implementations these days typically use a signed 64-bit integer. 704 1.1 christos Unsigned 32-bit integers are used on one or two platforms, 705 1.1 christos and 36-bit and 40-bit integers are also used occasionally. 706 1.1 christos Although earlier POSIX versions allowed <code>time_t</code> to be a 707 1.1 christos floating-point type, this was not supported by any practical 708 1.1 christos systems, and POSIX.1-2013 and the tz code both 709 1.1 christos require <code>time_t</code> 710 1.1 christos to be an integer type. 711 1.1 christos </li> 712 1.1 christos </ul> 713 1.1 christos <p> 714 1.1 christos These are the extensions that have been made to the POSIX functions: 715 1.1 christos </p> 716 1.1 christos <ul> 717 1.1 christos <li> 718 1.1 christos <p> 719 1.1 christos The TZ environment variable is used in generating the name of a file 720 1.1 christos from which time zone information is read (or is interpreted a la 721 1.1 christos POSIX); TZ is no longer constrained to be a three-letter time zone 722 1.1 christos name followed by a number of hours and an optional three-letter 723 1.1 christos daylight time zone name. The daylight saving time rules to be used 724 1.1 christos for a particular time zone are encoded in the time zone file; 725 1.1 christos the format of the file allows U.S., Australian, and other rules to be 726 1.1 christos encoded, and allows for situations where more than two time zone 727 1.1 christos abbreviations are used. 728 1.1 christos </p> 729 1.1 christos <p> 730 1.1 christos It was recognized that allowing the TZ environment variable to 731 1.1 christos take on values such as '<code>America/New_York</code>' might 732 1.1 christos cause "old" programs 733 1.1 christos (that expect TZ to have a certain form) to operate incorrectly; 734 1.1 christos consideration was given to using some other environment variable 735 1.1 christos (for example, TIMEZONE) to hold the string used to generate the 736 1.1 christos time zone information file name. In the end, however, it was decided 737 1.1 christos to continue using TZ: it is widely used for time zone purposes; 738 1.1 christos separately maintaining both TZ and TIMEZONE seemed a nuisance; 739 1.1 christos and systems where "new" forms of TZ might cause problems can simply 740 1.1 christos use TZ values such as "<code>EST5EDT</code>" which can be used both by 741 1.1 christos "new" programs (a la POSIX) and "old" programs (as zone names and 742 1.1 christos offsets). 743 1.1 christos </p> 744 1.1 christos </li> 745 1.1 christos <li> 746 1.1 christos The code supports platforms with a UT offset member 747 1.1 christos in <code>struct tm</code>, 748 1.1 christos e.g., <code>tm_gmtoff</code>. 749 1.1 christos </li> 750 1.1 christos <li> 751 1.1 christos The code supports platforms with a time zone abbreviation member in 752 1.1 christos <code>struct tm</code>, e.g., <code>tm_zone</code>. 753 1.1 christos </li> 754 1.1 christos <li> 755 1.1 christos Since the TZ environment variable can now be used to control time 756 1.1 christos conversion, the <code>daylight</code> 757 1.1 christos and <code>timezone</code> variables are no longer needed. 758 1.1 christos (These variables are defined and set by <code>tzset</code>; 759 1.1 christos however, their values will not be used 760 1.1 christos by <code>localtime</code>.) 761 1.1 christos </li> 762 1.1 christos <li> 763 1.1 christos Functions <code>tzalloc</code>, <code>tzfree</code>, 764 1.1 christos <code>localtime_rz</code>, and <code>mktime_z</code> for 765 1.1 christos more-efficient thread-safe applications that need to use 766 1.1 christos multiple time zones. The <code>tzalloc</code> 767 1.1 christos and <code>tzfree</code> functions allocate and free objects of 768 1.1 christos type <code>timezone_t</code>, and <code>localtime_rz</code> 769 1.1 christos and <code>mktime_z</code> are like <code>localtime_r</code> 770 1.1 christos and <code>mktime</code> with an extra 771 1.1 christos <code>timezone_t</code> argument. The functions were inspired 772 1.1 christos by NetBSD. 773 1.1 christos </li> 774 1.1 christos <li> 775 1.1 christos A function <code>tzsetwall</code> has been added to arrange 776 1.1 christos for the system's 777 1.1 christos best approximation to local wall clock time to be delivered by 778 1.1 christos subsequent calls to <code>localtime</code>. Source code for portable 779 1.1 christos applications that "must" run on local wall clock time should call 780 1.1 christos <code>tzsetwall</code>; if such code is moved to "old" systems that don't 781 1.1 christos provide tzsetwall, you won't be able to generate an executable program. 782 1.1 christos (These time zone functions also arrange for local wall clock time to be 783 1.1 christos used if tzset is called – directly or indirectly – 784 1.1 christos and there's no TZ 785 1.1 christos environment variable; portable applications should not, however, rely 786 1.1 christos on this behavior since it's not the way SVR2 systems behave.) 787 1.1 christos </li> 788 1.1 christos <li> 789 1.1 christos Negative <code>time_t</code> values are supported, on systems 790 1.1 christos where <code>time_t</code> is signed. 791 1.1 christos </li> 792 1.1 christos <li> 793 1.1 christos These functions can account for leap seconds, thanks to Bradley White. 794 1.1 christos </li> 795 1.1 christos </ul> 796 1.1 christos <p> 797 1.1 christos Points of interest to folks with other systems: 798 1.1 christos </p> 799 1.1 christos <ul> 800 1.1 christos <li> 801 1.1 christos Code compatible with this package is already part of many platforms, 802 1.1 christos including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS, 803 1.1 christos BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris. 804 1.1 christos On such hosts, the primary use of this package 805 1.1 christos is to update obsolete time zone rule tables. 806 1.1 christos To do this, you may need to compile the time zone compiler 807 1.1 christos '<code>zic</code>' supplied with this package instead of using 808 1.1 christos the system '<code>zic</code>', since the format 809 1.1 christos of <code>zic</code>'s input is occasionally extended, and a 810 1.1 christos platform may still be shipping an older <code>zic</code>. 811 1.1 christos </li> 812 1.1 christos <li> 813 1.1 christos The UNIX Version 7 <code>timezone</code> function is not 814 1.1 christos present in this package; 815 1.1 christos it's impossible to reliably map timezone's arguments (a "minutes west 816 1.1 christos of GMT" value and a "daylight saving time in effect" flag) to a 817 1.1 christos time zone abbreviation, and we refuse to guess. 818 1.1 christos Programs that in the past used the timezone function may now examine 819 1.1 christos <code>localtime(&clock)->tm_zone</code> 820 1.1 christos (if <code>TM_ZONE</code> is defined) or 821 1.1 christos <code>tzname[localtime(&clock)->tm_isdst]</code> 822 1.1 christos (if <code>HAVE_TZNAME</code> is defined) 823 1.1 christos to learn the correct time zone abbreviation to use. 824 1.1 christos </li> 825 1.1 christos <li> 826 1.1 christos The 4.2BSD <code>gettimeofday</code> function is not used in 827 1.1 christos this package. 828 1.1 christos This formerly let users obtain the current UTC offset and DST flag, 829 1.1 christos but this functionality was removed in later versions of BSD. 830 1.1 christos </li> 831 1.1 christos <li> 832 1.1 christos In SVR2, time conversion fails for near-minimum or near-maximum 833 1.1 christos <code>time_t</code> values when doing conversions for places 834 1.1 christos that don't use UT. 835 1.1 christos This package takes care to do these conversions correctly. 836 1.1 christos A comment in the source code tells how to get compatibly wrong 837 1.1 christos results. 838 1.1 christos </li> 839 1.1 christos </ul> 840 1.1 christos <p> 841 1.1 christos The functions that are conditionally compiled 842 1.1 christos if <code>STD_INSPIRED</code> is defined 843 1.1 christos should, at this point, be looked on primarily as food for thought. They are 844 1.1 christos not in any sense "standard compatible" – some are not, in fact, 845 1.1 christos specified in <em>any</em> standard. They do, however, represent responses of 846 1.1 christos various authors to 847 1.1 christos standardization proposals. 848 1.1 christos </p> 849 1.1 christos 850 1.1 christos <p> 851 1.1 christos Other time conversion proposals, in particular the one developed by folks at 852 1.1 christos Hewlett Packard, offer a wider selection of functions that provide capabilities 853 1.1 christos beyond those provided here. The absence of such functions from this package 854 1.1 christos is not meant to discourage the development, standardization, or use of such 855 1.1 christos functions. Rather, their absence reflects the decision to make this package 856 1.1 christos contain valid extensions to POSIX, to ensure its broad acceptability. If 857 1.1 christos more powerful time conversion functions can be standardized, so much the 858 1.1 christos better. 859 1.1 christos </p> 860 1.1 christos </section> 861 1.1 christos 862 1.1 christos 863 1.1 christos <section> 864 1.1 christos <h2 id="stability">Interface stability</h2> 865 1.1 christos <p> 866 1.1 christos The tz code and data supply the following interfaces: 867 1.1 christos </p> 868 1.1 christos <ul> 869 1.1 christos <li> 870 1.1 christos A set of zone names as per "<a href="#naming">Names of time zone 871 1.1 christos rules</a>" above. 872 1.1 christos </li> 873 1.1 christos <li> 874 1.1 christos Library functions described in "<a href="#functions">Time and date 875 1.1 christos functions</a>" above. 876 1.1 christos </li> 877 1.1 christos <li> 878 1.1 christos The programs <code>tzselect</code>, <code>zdump</code>, 879 1.1 christos and <code>zic</code>, documented in their man pages. 880 1.1 christos </li> 881 1.1 christos <li> 882 1.1 christos The format of <code>zic</code> input files, documented in 883 1.1 christos the <code>zic</code> man page. 884 1.1 christos </li> 885 1.1 christos <li> 886 1.1 christos The format of <code>zic</code> output files, documented in 887 1.1 christos the <code>tzfile</code> man page. 888 1.1 christos </li> 889 1.1 christos <li> 890 1.1 christos The format of zone table files, documented in <code>zone1970.tab</code>. 891 1.1 christos </li> 892 1.1 christos <li> 893 1.1 christos The format of the country code file, documented in <code>iso3166.tab</code>. 894 1.1 christos </li> 895 1.1 christos <li> 896 1.1 christos The version number of the code and data, as the first line of 897 1.1 christos the text file '<code>version</code>' in each release. 898 1.1 christos </li> 899 1.1 christos </ul> 900 1.1 christos <p> 901 1.1 christos Interface changes in a release attempt to preserve compatibility with 902 1.1 christos recent releases. For example, tz data files typically do not rely on 903 1.1 christos recently-added <code>zic</code> features, so that users can run 904 1.1 christos older <code>zic</code> versions to process newer data 905 1.1 christos files. <a href="tz-link.htm">Sources for time zone and daylight 906 1.1 christos saving time data</a> describes how 907 1.1 christos releases are tagged and distributed. 908 1.1 christos </p> 909 1.1 christos 910 1.1 christos <p> 911 1.1 christos Interfaces not listed above are less stable. For example, users 912 1.1 christos should not rely on particular UT offsets or abbreviations for 913 1.1 christos timestamps, as data entries are often based on guesswork and these 914 1.1 christos guesses may be corrected or improved. 915 1.1 christos </p> 916 1.1 christos </section> 917 1.1 christos 918 1.1 christos 919 1.1 christos <section> 920 1.1 christos <h2 id="calendar">Calendrical issues</h2> 921 1.1 christos <p> 922 1.1 christos Calendrical issues are a bit out of scope for a time zone database, 923 1.1 christos but they indicate the sort of problems that we would run into if we 924 1.1 christos extended the time zone database further into the past. An excellent 925 1.1 christos resource in this area is Nachum Dershowitz and Edward M. Reingold, 926 1.1 christos <cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical 927 1.1 christos Calculations: Third Edition</a></cite>, Cambridge University Press (2008). 928 1.1 christos Other information and sources are given in the file '<samp>calendars</samp>' 929 1.1 christos in the tz distribution. They sometimes disagree. 930 1.1 christos </p> 931 1.1 christos </section> 932 1.1 christos 933 1.1 christos 934 1.1 christos <section> 935 1.1 christos <h2 id="planets">Time and time zones on other planets</h2> 936 1.1 christos <p> 937 1.1 christos Some people's work schedules use Mars time. Jet Propulsion Laboratory 938 1.1 christos (JPL) coordinators have kept Mars time on and off at least since 1997 939 1.1 christos for the Mars Pathfinder mission. Some of their family members have 940 1.1 christos also adapted to Mars time. Dozens of special Mars watches were built 941 1.1 christos for JPL workers who kept Mars time during the Mars Exploration 942 1.1 christos Rovers mission (2004). These timepieces look like normal Seikos and 943 1.1 christos Citizens but use Mars seconds rather than terrestrial seconds. 944 1.1 christos </p> 945 1.1 christos 946 1.1 christos <p> 947 1.1 christos A Mars solar day is called a "sol" and has a mean period equal to 948 1.1 christos about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is 949 1.1 christos divided into a conventional 24-hour clock, so each Mars second equals 950 1.1 christos about 1.02749125 terrestrial seconds. 951 1.1 christos </p> 952 1.1 christos 953 1.1 christos <p> 954 1.1 christos The prime meridian of Mars goes through the center of the crater 955 1.1 christos Airy-0, named in honor of the British astronomer who built the 956 1.1 christos Greenwich telescope that defines Earth's prime meridian. Mean solar 957 1.1 christos time on the Mars prime meridian is called Mars Coordinated Time (MTC). 958 1.1 christos </p> 959 1.1 christos 960 1.1 christos <p> 961 1.1 christos Each landed mission on Mars has adopted a different reference for 962 1.1 christos solar time keeping, so there is no real standard for Mars time zones. 963 1.1 christos For example, the Mars Exploration Rover project (2004) defined two 964 1.1 christos time zones "Local Solar Time A" and "Local Solar Time B" for its two 965 1.1 christos missions, each zone designed so that its time equals local true solar 966 1.1 christos time at approximately the middle of the nominal mission. Such a "time 967 1.1 christos zone" is not particularly suited for any application other than the 968 1.1 christos mission itself. 969 1.1 christos </p> 970 1.1 christos 971 1.1 christos <p> 972 1.1 christos Many calendars have been proposed for Mars, but none have achieved 973 1.1 christos wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a 974 1.1 christos sequential count of Mars solar days elapsed since about 1873-12-29 975 1.1 christos 12:00 GMT. 976 1.1 christos </p> 977 1.1 christos 978 1.1 christos <p> 979 1.1 christos In our solar system, Mars is the planet with time and calendar most 980 1.1 christos like Earth's. On other planets, Sun-based time and calendars would 981 1.1 christos work quite differently. For example, although Mercury's sidereal 982 1.1 christos rotation period is 58.646 Earth days, Mercury revolves around the Sun 983 1.1 christos so rapidly that an observer on Mercury's equator would see a sunrise 984 1.1 christos only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury 985 1.1 christos day. Venus is more complicated, partly because its rotation is 986 1.1 christos slightly retrograde: its year is 1.92 of its days. Gas giants like 987 1.1 christos Jupiter are trickier still, as their polar and equatorial regions 988 1.1 christos rotate at different rates, so that the length of a day depends on 989 1.1 christos latitude. This effect is most pronounced on Neptune, where the day is 990 1.1 christos about 12 hours at the poles and 18 hours at the equator. 991 1.1 christos </p> 992 1.1 christos 993 1.1 christos <p> 994 1.1 christos Although the tz database does not support time on other planets, it is 995 1.1 christos documented here in the hopes that support will be added eventually. 996 1.1 christos </p> 997 1.1 christos 998 1.1 christos <p> 999 1.1 christos Sources: 1000 1.1 christos </p> 1001 1.1 christos <ul> 1002 1.1 christos <li> 1003 1.1 christos Michael Allison and Robert Schmunk, 1004 1.1 christos "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical 1005 1.1 christos Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" 1006 1.1 christos (2012-08-08). 1007 1.1 christos </li> 1008 1.1 christos <li> 1009 1.1 christos Jia-Rui Chong, 1010 1.1 christos "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays 1011 1.1 christos Fit for a Martian</a>", Los Angeles Times 1012 1.1 christos (2004-01-14), pp A1, A20-A21. 1013 1.1 christos </li> 1014 1.1 christos <li> 1015 1.1 christos Tom Chmielewski, 1016 1.1 christos "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet 1017 1.1 christos Lag Is Worse on Mars</a>", The Atlantic (2015-02-26) 1018 1.1 christos </li> 1019 1.1 christos <li> 1020 1.1 christos Matt Williams, 1021 1.1 christos "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How 1022 1.1 christos long is a day on the other planets of the solar system?</a>" 1023 1.1 christos (2017-04-27). 1024 1.1 christos </li> 1025 1.1 christos </ul> 1026 1.1 christos </section> 1027 1.1 christos 1028 1.1 christos <footer> 1029 1.1 christos <hr> 1030 1.1 christos This file is in the public domain, so clarified as of 2009-05-17 by 1031 1.1 christos Arthur David Olson. 1032 1.1 christos </footer> 1033 1.1 christos </body> 1034 1.1 christos </html> 1035
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