Theory revision 1.5
1 # $NetBSD: Theory,v 1.5 2000/12/12 15:25:41 kleink Exp $ 2 @(#)Theory 7.11 3 4 5 ----- Outline ----- 6 7 Time and date functions 8 Names of time zone regions 9 Time zone abbreviations 10 Calendrical issues 11 12 13 ----- Time and date functions ----- 14 15 These time and date functions are upwards compatible with POSIX.1, 16 an international standard for Unix-like systems. 17 As of this writing, the current edition of POSIX.1 is: 18 19 Information technology --Portable Operating System Interface (POSIX (R)) 20 -- Part 1: System Application Program Interface (API) [C Language] 21 ISO/IEC 9945-1:1996 22 ANSI/IEEE Std 1003.1, 1996 Edition 23 1996-07-12 24 25 POSIX.1 has the following properties and limitations. 26 27 * In POSIX.1, time display in a process is controlled by the 28 environment variable TZ. Unfortunately, the POSIX.1 TZ string takes 29 a form that is hard to describe and is error-prone in practice. 30 Also, POSIX.1 TZ strings can't deal with other (for example, Israeli) 31 daylight saving time rules, or situations where more than two 32 time zone abbreviations are used in an area. 33 34 The POSIX.1 TZ string takes the following form: 35 36 stdoffset[dst[offset],date[/time],date[/time]] 37 38 where: 39 40 std and dst 41 are 3 or more characters specifying the standard 42 and daylight saving time (DST) zone names. 43 offset 44 is of the form `[-]hh:[mm[:ss]]' and specifies the 45 offset west of UTC. The default DST offset is one hour 46 ahead of standard time. 47 date[/time],date[/time] 48 specifies the beginning and end of DST. If this is absent, 49 the system supplies its own rules for DST, and these can 50 differ from year to year; typically US DST rules are used. 51 time 52 takes the form `hh:[mm[:ss]]' and defaults to 02:00. 53 date 54 takes one of the following forms: 55 Jn (1<=n<=365) 56 origin-1 day number not counting February 29 57 n (0<=n<=365) 58 origin-0 day number counting February 29 if present 59 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 60 for the dth day of week n of month m of the year, 61 where week 1 is the first week in which day d appears, 62 and `5' stands for the last week in which day d appears 63 (which may be either the 4th or 5th week). 64 65 * In POSIX.1, when a TZ value like "EST5EDT" is parsed, 66 typically the current US DST rules are used, 67 but this means that the US DST rules are compiled into each program 68 that does time conversion. This means that when US time conversion 69 rules change (as in the United States in 1987), all programs that 70 do time conversion must be recompiled to ensure proper results. 71 72 * In POSIX.1, there's no tamper-proof way for a process to learn the 73 system's best idea of local wall clock. (This is important for 74 applications that an administrator wants used only at certain times-- 75 without regard to whether the user has fiddled the "TZ" environment 76 variable. While an administrator can "do everything in UTC" to get 77 around the problem, doing so is inconvenient and precludes handling 78 daylight saving time shifts--as might be required to limit phone 79 calls to off-peak hours.) 80 81 * POSIX.1 requires that systems ignore leap seconds. 82 83 These are the extensions that have been made to the POSIX.1 functions: 84 85 * The "TZ" environment variable is used in generating the name of a file 86 from which time zone information is read (or is interpreted a la 87 POSIX); "TZ" is no longer constrained to be a three-letter time zone 88 name followed by a number of hours and an optional three-letter 89 daylight time zone name. The daylight saving time rules to be used 90 for a particular time zone are encoded in the time zone file; 91 the format of the file allows U.S., Australian, and other rules to be 92 encoded, and allows for situations where more than two time zone 93 abbreviations are used. 94 95 It was recognized that allowing the "TZ" environment variable to 96 take on values such as "America/New_York" might cause "old" programs 97 (that expect "TZ" to have a certain form) to operate incorrectly; 98 consideration was given to using some other environment variable 99 (for example, "TIMEZONE") to hold the string used to generate the 100 time zone information file name. In the end, however, it was decided 101 to continue using "TZ": it is widely used for time zone purposes; 102 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 103 and systems where "new" forms of "TZ" might cause problems can simply 104 use TZ values such as "EST5EDT" which can be used both by 105 "new" programs (a la POSIX) and "old" programs (as zone names and 106 offsets). 107 108 * To handle places where more than two time zone abbreviations are used, 109 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 110 (where "tmp" is the value the function returns) to the time zone 111 abbreviation to be used. This differs from POSIX.1, where the elements 112 of tzname are only changed as a result of calls to tzset. 113 114 * Since the "TZ" environment variable can now be used to control time 115 conversion, the "daylight" and "timezone" variables are no longer 116 needed. (These variables are defined and set by "tzset"; however, their 117 values will not be used by "localtime.") 118 119 * The "localtime" function has been set up to deliver correct results 120 for near-minimum or near-maximum time_t values. (A comment in the 121 source code tells how to get compatibly wrong results). 122 123 * A function "tzsetwall" has been added to arrange for the system's 124 best approximation to local wall clock time to be delivered by 125 subsequent calls to "localtime." Source code for portable 126 applications that "must" run on local wall clock time should call 127 "tzsetwall();" if such code is moved to "old" systems that don't 128 provide tzsetwall, you won't be able to generate an executable program. 129 (These time zone functions also arrange for local wall clock time to be 130 used if tzset is called--directly or indirectly--and there's no "TZ" 131 environment variable; portable applications should not, however, rely 132 on this behavior since it's not the way SVR2 systems behave.) 133 134 * These functions can account for leap seconds, thanks to Bradley White 135 (bww (a] k.cs.cmu.edu). 136 137 Points of interest to folks with other systems: 138 139 * This package is already part of many POSIX-compliant hosts, 140 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 141 On such hosts, the primary use of this package 142 is to update obsolete time zone rule tables. 143 To do this, you may need to compile the time zone compiler 144 `zic' supplied with this package instead of using the system `zic', 145 since the format of zic's input changed slightly in late 1994, 146 and many vendors still do not support the new input format. 147 148 * The Unix Version 7 "timezone" function is not present in this package; 149 it's impossible to reliably map timezone's arguments (a "minutes west 150 of GMT" value and a "daylight saving time in effect" flag) to a 151 time zone abbreviation, and we refuse to guess. 152 Programs that in the past used the timezone function may now examine 153 tzname[localtime(&clock)->tm_isdst] to learn the correct time 154 zone abbreviation to use. Alternatively, use 155 localtime(&clock)->tm_zone if this has been enabled. 156 157 * The 4.2BSD gettimeofday function is not used in this package. 158 This formerly let users obtain the current UTC offset and DST flag, 159 but this functionality was removed in later versions of BSD. 160 161 * In SVR2, time conversion fails for near-minimum or near-maximum 162 time_t values when doing conversions for places that don't use UTC. 163 This package takes care to do these conversions correctly. 164 165 The functions that are conditionally compiled if STD_INSPIRED is defined 166 should, at this point, be looked on primarily as food for thought. They are 167 not in any sense "standard compatible"--some are not, in fact, specified in 168 *any* standard. They do, however, represent responses of various authors to 169 standardization proposals. 170 171 Other time conversion proposals, in particular the one developed by folks at 172 Hewlett Packard, offer a wider selection of functions that provide capabilities 173 beyond those provided here. The absence of such functions from this package 174 is not meant to discourage the development, standardization, or use of such 175 functions. Rather, their absence reflects the decision to make this package 176 contain valid extensions to POSIX.1, to ensure its broad 177 acceptability. If more powerful time conversion functions can be standardized, 178 so much the better. 179 180 181 ----- Names of time zone rule files ----- 182 183 The names of this package's installed time zone rule files are chosen to 184 help minimize possible future incompatibilities due to political events. 185 Ordinarily, names of countries are not used, to avoid incompatibilities 186 when countries change their name (e.g. Zaire->Congo) or 187 when locations change countries (e.g. Hong Kong from UK colony to China). 188 189 Names normally have the form AREA/LOCATION, where AREA is the name 190 of a continent or ocean, and LOCATION is the name of a specific 191 location within that region. North and South America share the same 192 area, `America'. Typical names are `Africa/Cairo', `America/New_York', 193 and `Pacific/Honolulu'. 194 195 Here are the general rules used for choosing location names, 196 in decreasing order of importance: 197 198 Use only valid Posix file names. Use only Ascii letters, digits, `.', 199 `-' and `_'. Do not exceed 14 characters or start with `-'. 200 E.g. prefer `Brunei' to `Bandar_Seri_Begawan'. 201 Include at least one location per time zone rule set per country. 202 One such location is enough. Use ISO 3166 (see the file 203 iso3166.tab) to help decide whether something is a country. 204 If all the clocks in a country's region have agreed since 1970, 205 don't bother to include more than one location 206 even if subregions' clocks disagreed before 1970. 207 Otherwise these tables would become annoyingly large. 208 If a name is ambiguous, use a less ambiguous alternative; 209 e.g. many cities are named San Jose and Georgetown, so 210 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'. 211 Keep locations compact. Use cities or small islands, not countries 212 or regions, so that any future time zone changes do not split 213 locations into different time zones. E.g. prefer `Paris' 214 to `France', since France has had multiple time zones. 215 Use traditional English spelling, e.g. prefer `Rome' to `Roma', and 216 prefer `Athens' to the true name (which uses Greek letters). 217 The Posix file name restrictions encourage this rule. 218 Use the most populous among locations in a country's time zone, 219 e.g. prefer `Shanghai' to `Beijing'. Among locations with 220 similar populations, pick the best-known location, 221 e.g. prefer `Rome' to `Milan'. 222 Use the singular form, e.g. prefer `Canary' to `Canaries'. 223 Omit common suffixes like `_Islands' and `_City', unless that 224 would lead to ambiguity. E.g. prefer `Cayman' to 225 `Cayman_Islands' and `Guatemala' to `Guatemala_City', 226 but prefer `Mexico_City' to `Mexico' because the country 227 of Mexico has several time zones. 228 Use `_' to represent a space. 229 Omit `.' from abbreviations in names, e.g. prefer `St_Helena' 230 to `St._Helena'. 231 Do not change established names if they only marginally 232 violate the above rules. For example, don't change 233 the existing name `Rome' to `Milan' merely because 234 Milan's population has grown to be somewhat greater 235 than Rome's. 236 If a name is changed, put its old spelling in the `backward' file. 237 238 The file `zone.tab' lists the geographical locations used to name 239 time zone rule files. 240 241 Older versions of this package used a different naming scheme, 242 and these older names are still supported. 243 See the file `backward' for most of these older names 244 (e.g. `US/Eastern' instead of `America/New_York'). 245 The other old-fashioned names still supported are 246 `WET', `CET', `MET', `EET' (see the file `europe'), 247 and `Factory' (see the file `factory'). 248 249 250 ----- Time zone abbreviations ----- 251 252 When this package is installed, it generates time zone abbreviations 253 like `EST' to be compatible with human tradition and POSIX.1. 254 Here are the general rules used for choosing time zone abbreviations, 255 in decreasing order of importance: 256 257 Use abbreviations that consist of 3 or more upper-case Ascii letters, 258 except use "___" for locations while uninhabited. 259 Posix.1 requires at least 3 characters, and the restriction to 260 upper-case Ascii letters follows most traditions. 261 Previous editions of this database also used characters like 262 ' ' and '?', but these characters have a special meaning to 263 the shell and cause commands like 264 set `date` 265 to have unexpected effects. In theory, the character set could 266 be !%./@A-Z^_a-z{}, but these tables use only upper-case 267 Ascii letters (and "___"). 268 Use abbreviations that are in common use among English-speakers, 269 e.g. `EST' for Eastern Standard Time in North America. 270 We assume that applications translate them to other languages 271 as part of the normal localization process; for example, 272 a French application might translate `EST' to `HNE'. 273 For zones whose times are taken from a city's longitude, use the 274 traditional xMT notation, e.g. `PMT' for Paris Mean Time. 275 The only name like this in current use is `GMT'. 276 If there is no common English abbreviation, abbreviate the English 277 translation of the usual phrase used by native speakers. 278 If this is not available or is a phrase mentioning the country 279 (e.g. ``Cape Verde Time''), then: 280 281 When a country has a single or principal time zone region, 282 append `T' to the country's ISO code, e.g. `CVT' for 283 Cape Verde Time. For summer time append `ST'; 284 for double summer time append `DST'; etc. 285 When a country has multiple time zones, take the first three 286 letters of an English place name identifying each zone 287 and then append `T', `ST', etc. as before; 288 e.g. `VLAST' for VLAdivostok Summer Time. 289 290 Application writers should note that these abbreviations are ambiguous 291 in practice: e.g. `EST' has a different meaning in Australia than 292 it does in the United States. In new applications, it's often better 293 to use numeric UTC offsets like `-0500' instead of time zone 294 abbreviations like `EST'; this avoids the ambiguity. 295 296 297 ----- Calendrical issues ----- 298 299 Calendrical issues are a bit out of scope for a time zone database, 300 but they indicate the sort of problems that we would run into if we 301 extended the time zone database further into the past. An excellent 302 resource in this area is Nachum Dershowitz and Edward M. Reingold, 303 <a href="http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml"> 304 Calendrical Calculations 305 </a>, Cambridge University Press (1997). Other information and 306 sources are given below. They sometimes disagree. 307 308 309 France 310 311 Gregorian calendar adopted 1582-12-20. 312 French Revolutionary calendar used 1793-11-24 through 1805-12-31, 313 and (in Paris only) 1871-05-06 through 1871-05-23. 314 315 316 Russia 317 318 From Chris Carrier <72157.3334 (a] CompuServe.COM> (1996-12-02): 319 On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar'' 320 with 30-day months plus 5 holidays, with a 5-day week. 321 On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 322 Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 323 reverted to the 7-day week. With the 6-day week the usual days 324 off were the 6th, 12th, 18th, 24th and 30th of the month. 325 (Source: Evitiar Zerubavel, _The Seven Day Circle_) 326 327 328 Mark Brader reported a similar story in "The Book of Calendars", edited 329 by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 330 331 From: Petteri Sulonen (via Usenet) 332 Date: 14 Jan 1999 00:00:00 GMT 333 Message-ID: <Petteri.Sulonen-1401991626030001 (a] lapin-kulta.in.helsinki.fi> 334 335 If your source is correct, how come documents between 1929 -- 1940 were 336 still dated using the conventional, Gregorian calendar? 337 338 I can post a scan of a document dated December 1, 1934, signed by 339 Yenukidze, the secretary, on behalf of Kalinin, the President of the 340 Executive Committee of the Supreme Soviet, if you like. 341 342 343 344 Sweden (and Finland) 345 346 From: msb (a] sq.com (Mark Brader) 347 <a href="news:1996Jul6.012937.29190 (a] sq.com"> 348 Subject: Re: Gregorian reform -- a part of locale? 349 </a> 350 Date: 1996-07-06 351 352 In 1700, Denmark made the transition from Julian to Gregorian. Sweden 353 decided to *start* a transition in 1700 as well, but rather than have one of 354 those unsightly calendar gaps :-), they simply decreed that the next leap 355 year after 1696 would be in 1744 -- putting the whole country on a calendar 356 different from both Julian and Gregorian for a period of 40 years. 357 358 However, in 1704 something went wrong and the plan was not carried through; 359 they did, after all, have a leap year that year. And one in 1708. In 1712 360 they gave it up and went back to Julian, putting 30 days in February that 361 year!... 362 363 Then in 1753, Sweden made the transition to Gregorian in the usual manner, 364 getting there only 13 years behind the original schedule. 365 366 (A previous posting of this story was challenged, and Swedish readers 367 produced the following references to support it: "Tiderakning och historia" 368 by Natanael Beckman (1924) and "Tid, en bok om tiderakning och 369 kalendervasen" by Lars-Olof Lode'n (no date was given).) 370 371 372 Grotefend's data 373 374 From: "Michael Palmer" <mpalmer (a] netcom.com> [with one obvious typo fixed] 375 Subject: Re: Gregorian Calendar (was Re: Another FHC related question 376 Newsgroups: soc.genealogy.german 377 Date: Tue, 9 Feb 1999 02:32:48 -800 378 Message-ID: <199902091032.CAA09644 (a] netcom10.netcom.com> 379 380 The following is a(n incomplete) listing, arranged chronologically, of 381 European states, with the date they converted from the Julian to the 382 Gregorian calendar: 383 384 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 385 Catholics and Danzig only) 386 09/20 Dec 1582 - France, Lorraine 387 388 21 Dec 1582/ 389 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 390 10/21 Feb 1583 - bishopric of Liege (L"uttich) 391 13/24 Feb 1583 - bishopric of Augsburg 392 04/15 Oct 1583 - electorate of Trier 393 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 394 Salzburg, Brixen 395 13/24 Oct 1583 - Austrian Oberelsass and Breisgau 396 20/31 Oct 1583 - bishopric of Basel 397 02/13 Nov 1583 - duchy of J"ulich-Berg 398 02/13 Nov 1583 - electorate and city of K"oln 399 04/15 Nov 1583 - bishopric of W"urzburg 400 11/22 Nov 1583 - electorate of Mainz 401 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 402 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve 403 14/25 Dec 1583 - Steiermark 404 405 06/17 Jan 1584 - Austria and Bohemia 406 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn 407 12/23 Jan 1584 - Silesia and the Lausitz 408 22 Jan/ 409 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 410 Jun 1584 - Unterwalden 411 01/12 Jul 1584 - duchy of Westfalen 412 413 16/27 Jun 1585 - bishopric of Paderborn 414 415 14/25 Dec 1590 - Transylvania 416 417 22 Aug/ 418 02 Sep 1612 - duchy of Prussia 419 420 13/24 Dec 1614 - Pfalz-Neuburg 421 422 1617 - duchy of Kurland (reverted to the Julian calendar in 423 1796) 424 425 1624 - bishopric of Osnabr"uck 426 427 1630 - bishopric of Minden 428 429 15/26 Mar 1631 - bishopric of Hildesheim 430 431 1655 - Kanton Wallis 432 433 05/16 Feb 1682 - city of Strassburg 434 435 18 Feb/ 436 01 Mar 1700 - Protestant Germany (including Swedish possessions in 437 Germany), Denmark, Norway 438 30 Jun/ 439 12 Jul 1700 - Gelderland, Zutphen 440 10 Nov/ 441 12 Dec 1700 - Utrecht, Overijssel 442 443 31 Dec 1700/ 444 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva, 445 Turgau, and Schaffhausen 446 447 1724 - Glarus, Appenzell, and the city of St. Gallen 448 449 01 Jan 1750 - Pisa and Florence 450 451 02/14 Sep 1752 - Great Britain 452 453 17 Feb/ 454 01 Mar 1753 - Sweden 455 456 1760-1812 - Graub"unden 457 458 The Russian empire (including Finland and the Baltic states) did not 459 convert to the Gregorian calendar until the Soviet revolution of 1917. 460 461 Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 462 Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 463 (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 464
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