Theory revision 1.6
1# $NetBSD: Theory,v 1.6 2002/01/29 12:40:33 kleink Exp $ 2@(#)Theory 7.13 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 15These time and date functions are upwards compatible with POSIX.1, 16an international standard for Unix-like systems. 17As 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 25POSIX.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 83These 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@k.cs.cmu.edu). 136 137Points 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 165The functions that are conditionally compiled if STD_INSPIRED is defined 166should, at this point, be looked on primarily as food for thought. They are 167not in any sense "standard compatible"--some are not, in fact, specified in 168*any* standard. They do, however, represent responses of various authors to 169standardization proposals. 170 171Other time conversion proposals, in particular the one developed by folks at 172Hewlett Packard, offer a wider selection of functions that provide capabilities 173beyond those provided here. The absence of such functions from this package 174is not meant to discourage the development, standardization, or use of such 175functions. Rather, their absence reflects the decision to make this package 176contain valid extensions to POSIX.1, to ensure its broad 177acceptability. If more powerful time conversion functions can be standardized, 178so much the better. 179 180 181----- Names of time zone rule files ----- 182 183The time zone rule file naming conventions attempt to strike a balance 184among the following goals: 185 186 * Uniquely identify every national region where clocks have all 187 agreed since 1970. This is essential for the intended use: static 188 clocks keeping local civil time. 189 190 * Indicate to humans as to where that region is. This simplifes use. 191 192 * Be robust in the presence of political changes. This reduces the 193 number of updates and backward-compatibility hacks. For example, 194 names of countries are ordinarily not used, to avoid 195 incompatibilities when countries change their name 196 (e.g. Zaire->Congo) or when locations change countries 197 (e.g. Hong Kong from UK colony to China). 198 199 * Be portable to a wide variety of implementations. 200 This promotes use of the technology. 201 202 * Use a consistent naming convention over the entire world. 203 This simplifies both use and maintenance. 204 205This naming convention is not intended for use by inexperienced users 206to select TZ values by themselves (though they can of course examine 207and reuse existing settings). Distributors should provide 208documentation and/or a simple selection interface that explains the 209names; see the 'tzselect' program supplied with this distribution for 210one example. 211 212Names normally have the form AREA/LOCATION, where AREA is the name 213of a continent or ocean, and LOCATION is the name of a specific 214location within that region. North and South America share the same 215area, `America'. Typical names are `Africa/Cairo', `America/New_York', 216and `Pacific/Honolulu'. 217 218Here are the general rules used for choosing location names, 219in decreasing order of importance: 220 221 Use only valid POSIX file name components (i.e., the parts of 222 names other than `/'). Within a file name component, 223 use only ASCII letters, `.', `-' and `_'. Do not use 224 digits, as that might create an ambiguity with POSIX 225 TZ strings. A file name component must not exceed 14 226 characters or start with `-'. E.g., prefer `Brunei' 227 to `Bandar_Seri_Begawan'. 228 Include at least one location per time zone rule set per country. 229 One such location is enough. Use ISO 3166 (see the file 230 iso3166.tab) to help decide whether something is a country. 231 If all the clocks in a country's region have agreed since 1970, 232 don't bother to include more than one location 233 even if subregions' clocks disagreed before 1970. 234 Otherwise these tables would become annoyingly large. 235 If a name is ambiguous, use a less ambiguous alternative; 236 e.g. many cities are named San Jose and Georgetown, so 237 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'. 238 Keep locations compact. Use cities or small islands, not countries 239 or regions, so that any future time zone changes do not split 240 locations into different time zones. E.g. prefer `Paris' 241 to `France', since France has had multiple time zones. 242 Use mainstream English spelling, e.g. prefer `Rome' to `Roma', and 243 prefer `Athens' to the true name (which uses Greek letters). 244 The POSIX file name restrictions encourage this rule. 245 Use the most populous among locations in a country's time zone, 246 e.g. prefer `Shanghai' to `Beijing'. Among locations with 247 similar populations, pick the best-known location, 248 e.g. prefer `Rome' to `Milan'. 249 Use the singular form, e.g. prefer `Canary' to `Canaries'. 250 Omit common suffixes like `_Islands' and `_City', unless that 251 would lead to ambiguity. E.g. prefer `Cayman' to 252 `Cayman_Islands' and `Guatemala' to `Guatemala_City', 253 but prefer `Mexico_City' to `Mexico' because the country 254 of Mexico has several time zones. 255 Use `_' to represent a space. 256 Omit `.' from abbreviations in names, e.g. prefer `St_Helena' 257 to `St._Helena'. 258 Do not change established names if they only marginally 259 violate the above rules. For example, don't change 260 the existing name `Rome' to `Milan' merely because 261 Milan's population has grown to be somewhat greater 262 than Rome's. 263 If a name is changed, put its old spelling in the `backward' file. 264 265The file `zone.tab' lists the geographical locations used to name 266time zone rule files. 267 268Older versions of this package used a different naming scheme, 269and these older names are still supported. 270See the file `backward' for most of these older names 271(e.g. `US/Eastern' instead of `America/New_York'). 272The other old-fashioned names still supported are 273`WET', `CET', `MET', `EET' (see the file `europe'), 274and `Factory' (see the file `factory'). 275 276 277----- Time zone abbreviations ----- 278 279When this package is installed, it generates time zone abbreviations 280like `EST' to be compatible with human tradition and POSIX.1. 281Here are the general rules used for choosing time zone abbreviations, 282in decreasing order of importance: 283 284 Use abbreviations that consist of three or more ASCII letters. 285 Previous editions of this database also used characters like 286 ' ' and '?', but these characters have a special meaning to 287 the shell and cause commands like 288 set `date` 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 the rule has been relaxed. 293 294 This rule guarantees that all abbreviations could have 295 been specified by a POSIX.1 TZ string. POSIX.1 296 requires at least three characters for an 297 abbreviation. POSIX.1-1996 says that an abbreviation 298 cannot start with ':', and cannot contain ',', '-', 299 '+', NUL, or a digit. Draft 7 of POSIX 1003.1-200x 300 changes this rule to say that an abbreviation can 301 contain only '-', '+', and alphanumeric characters in 302 the current locale. To be portable to both sets of 303 rules, an abbreviation must therefore use only ASCII 304 letters, as these are the only letters that are 305 alphabetic in all locales. 306 307 Use abbreviations that are in common use among English-speakers, 308 e.g. `EST' for Eastern Standard Time in North America. 309 We assume that applications translate them to other languages 310 as part of the normal localization process; for example, 311 a French application might translate `EST' to `HNE'. 312 313 For zones whose times are taken from a city's longitude, use the 314 traditional xMT notation, e.g. `PMT' for Paris Mean Time. 315 The only name like this in current use is `GMT'. 316 317 If there is no common English abbreviation, abbreviate the English 318 translation of the usual phrase used by native speakers. 319 If this is not available or is a phrase mentioning the country 320 (e.g. ``Cape Verde Time''), then: 321 322 When a country has a single or principal time zone region, 323 append `T' to the country's ISO code, e.g. `CVT' for 324 Cape Verde Time. For summer time append `ST'; 325 for double summer time append `DST'; etc. 326 When a country has multiple time zones, take the first three 327 letters of an English place name identifying each zone 328 and then append `T', `ST', etc. as before; 329 e.g. `VLAST' for VLAdivostok Summer Time. 330 331 Use "zzz" for locations while uninhabited. The mnemonic is that 332 these locations are, in some sense, asleep. 333 334Application writers should note that these abbreviations are ambiguous 335in practice: e.g. `EST' has a different meaning in Australia than 336it does in the United States. In new applications, it's often better 337to use numeric UTC offsets like `-0500' instead of time zone 338abbreviations like `EST'; this avoids the ambiguity. 339 340 341----- Calendrical issues ----- 342 343Calendrical issues are a bit out of scope for a time zone database, 344but they indicate the sort of problems that we would run into if we 345extended the time zone database further into the past. An excellent 346resource in this area is Nachum Dershowitz and Edward M. Reingold, 347<a href="http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml"> 348Calendrical Calculations 349</a>, Cambridge University Press (1997). Other information and 350sources are given below. They sometimes disagree. 351 352 353France 354 355Gregorian calendar adopted 1582-12-20. 356French Revolutionary calendar used 1793-11-24 through 1805-12-31, 357and (in Paris only) 1871-05-06 through 1871-05-23. 358 359 360Russia 361 362From Chris Carrier <72157.3334@CompuServe.COM> (1996-12-02): 363On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar'' 364with 30-day months plus 5 holidays, with a 5-day week. 365On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the 366Gregorian calendar while retaining the 6-day week; on 1940-06-27 it 367reverted to the 7-day week. With the 6-day week the usual days 368off were the 6th, 12th, 18th, 24th and 30th of the month. 369(Source: Evitiar Zerubavel, _The Seven Day Circle_) 370 371 372Mark Brader reported a similar story in "The Book of Calendars", edited 373by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: 374 375From: Petteri Sulonen (via Usenet) 376Date: 14 Jan 1999 00:00:00 GMT 377Message-ID: <Petteri.Sulonen-1401991626030001@lapin-kulta.in.helsinki.fi> 378 379If your source is correct, how come documents between 1929 -- 1940 were 380still dated using the conventional, Gregorian calendar? 381 382I can post a scan of a document dated December 1, 1934, signed by 383Yenukidze, the secretary, on behalf of Kalinin, the President of the 384Executive Committee of the Supreme Soviet, if you like. 385 386 387 388Sweden (and Finland) 389 390From: msb@sq.com (Mark Brader) 391<a href="news:1996Jul6.012937.29190@sq.com"> 392Subject: Re: Gregorian reform -- a part of locale? 393</a> 394Date: 1996-07-06 395 396In 1700, Denmark made the transition from Julian to Gregorian. Sweden 397decided to *start* a transition in 1700 as well, but rather than have one of 398those unsightly calendar gaps :-), they simply decreed that the next leap 399year after 1696 would be in 1744 -- putting the whole country on a calendar 400different from both Julian and Gregorian for a period of 40 years. 401 402However, in 1704 something went wrong and the plan was not carried through; 403they did, after all, have a leap year that year. And one in 1708. In 1712 404they gave it up and went back to Julian, putting 30 days in February that 405year!... 406 407Then in 1753, Sweden made the transition to Gregorian in the usual manner, 408getting there only 13 years behind the original schedule. 409 410(A previous posting of this story was challenged, and Swedish readers 411produced the following references to support it: "Tiderakning och historia" 412by Natanael Beckman (1924) and "Tid, en bok om tiderakning och 413kalendervasen" by Lars-Olof Lode'n (no date was given).) 414 415 416Grotefend's data 417 418From: "Michael Palmer" <mpalmer@netcom.com> [with one obvious typo fixed] 419Subject: Re: Gregorian Calendar (was Re: Another FHC related question 420Newsgroups: soc.genealogy.german 421Date: Tue, 9 Feb 1999 02:32:48 -800 422Message-ID: <199902091032.CAA09644@netcom10.netcom.com> 423 424The following is a(n incomplete) listing, arranged chronologically, of 425European states, with the date they converted from the Julian to the 426Gregorian calendar: 427 42804/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman 429 Catholics and Danzig only) 43009/20 Dec 1582 - France, Lorraine 431 43221 Dec 1582/ 433 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 43410/21 Feb 1583 - bishopric of Liege (L"uttich) 43513/24 Feb 1583 - bishopric of Augsburg 43604/15 Oct 1583 - electorate of Trier 43705/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, 438 Salzburg, Brixen 43913/24 Oct 1583 - Austrian Oberelsass and Breisgau 44020/31 Oct 1583 - bishopric of Basel 44102/13 Nov 1583 - duchy of J"ulich-Berg 44202/13 Nov 1583 - electorate and city of K"oln 44304/15 Nov 1583 - bishopric of W"urzburg 44411/22 Nov 1583 - electorate of Mainz 44516/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 44617/28 Nov 1583 - bishopric of M"unster and duchy of Cleve 44714/25 Dec 1583 - Steiermark 448 44906/17 Jan 1584 - Austria and Bohemia 45011/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn 45112/23 Jan 1584 - Silesia and the Lausitz 45222 Jan/ 453 02 Feb 1584 - Hungary (legally on 21 Oct 1587) 454 Jun 1584 - Unterwalden 45501/12 Jul 1584 - duchy of Westfalen 456 45716/27 Jun 1585 - bishopric of Paderborn 458 45914/25 Dec 1590 - Transylvania 460 46122 Aug/ 462 02 Sep 1612 - duchy of Prussia 463 46413/24 Dec 1614 - Pfalz-Neuburg 465 466 1617 - duchy of Kurland (reverted to the Julian calendar in 467 1796) 468 469 1624 - bishopric of Osnabr"uck 470 471 1630 - bishopric of Minden 472 47315/26 Mar 1631 - bishopric of Hildesheim 474 475 1655 - Kanton Wallis 476 47705/16 Feb 1682 - city of Strassburg 478 47918 Feb/ 480 01 Mar 1700 - Protestant Germany (including Swedish possessions in 481 Germany), Denmark, Norway 48230 Jun/ 483 12 Jul 1700 - Gelderland, Zutphen 48410 Nov/ 485 12 Dec 1700 - Utrecht, Overijssel 486 48731 Dec 1700/ 488 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva, 489 Turgau, and Schaffhausen 490 491 1724 - Glarus, Appenzell, and the city of St. Gallen 492 49301 Jan 1750 - Pisa and Florence 494 49502/14 Sep 1752 - Great Britain 496 49717 Feb/ 498 01 Mar 1753 - Sweden 499 5001760-1812 - Graub"unden 501 502The Russian empire (including Finland and the Baltic states) did not 503convert to the Gregorian calendar until the Soviet revolution of 1917. 504 505Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen 506Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend 507(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. 508