]> git.saurik.com Git - apple/system_cmds.git/blob - zic.tproj/Theory
system_cmds-433.8.tar.gz
[apple/system_cmds.git] / zic.tproj / Theory
1 @(#)Theory 7.15
2
3
4 ----- Outline -----
5
6 Time and date functions
7 Names of time zone regions
8 Time zone abbreviations
9 Calendrical issues
10 Time and time zones on Mars
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@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 time zone rule file naming conventions attempt to strike a balance
184 among 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
205 This naming convention is not intended for use by inexperienced users
206 to select TZ values by themselves (though they can of course examine
207 and reuse existing settings). Distributors should provide
208 documentation and/or a simple selection interface that explains the
209 names; see the 'tzselect' program supplied with this distribution for
210 one example.
211
212 Names normally have the form AREA/LOCATION, where AREA is the name
213 of a continent or ocean, and LOCATION is the name of a specific
214 location within that region. North and South America share the same
215 area, `America'. Typical names are `Africa/Cairo', `America/New_York',
216 and `Pacific/Honolulu'.
217
218 Here are the general rules used for choosing location names,
219 in 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
265 The file `zone.tab' lists the geographical locations used to name
266 time zone rule files.
267
268 Older versions of this package used a different naming scheme,
269 and these older names are still supported.
270 See the file `backward' for most of these older names
271 (e.g. `US/Eastern' instead of `America/New_York').
272 The other old-fashioned names still supported are
273 `WET', `CET', `MET', `EET' (see the file `europe'),
274 and `Factory' (see the file `factory').
275
276
277 ----- Time zone abbreviations -----
278
279 When this package is installed, it generates time zone abbreviations
280 like `EST' to be compatible with human tradition and POSIX.1.
281 Here are the general rules used for choosing time zone abbreviations,
282 in 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
334 Application writers should note that these abbreviations are ambiguous
335 in practice: e.g. `EST' has a different meaning in Australia than
336 it does in the United States. In new applications, it's often better
337 to use numeric UTC offsets like `-0500' instead of time zone
338 abbreviations like `EST'; this avoids the ambiguity.
339
340
341 ----- Calendrical issues -----
342
343 Calendrical issues are a bit out of scope for a time zone database,
344 but they indicate the sort of problems that we would run into if we
345 extended the time zone database further into the past. An excellent
346 resource in this area is Nachum Dershowitz and Edward M. Reingold,
347 <a href="http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml">
348 Calendrical Calculations
349 </a>, Cambridge University Press (1997). Other information and
350 sources are given below. They sometimes disagree.
351
352
353 France
354
355 Gregorian calendar adopted 1582-12-20.
356 French Revolutionary calendar used 1793-11-24 through 1805-12-31,
357 and (in Paris only) 1871-05-06 through 1871-05-23.
358
359
360 Russia
361
362 From Chris Carrier <72157.3334@CompuServe.COM> (1996-12-02):
363 On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar''
364 with 30-day months plus 5 holidays, with a 5-day week.
365 On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
366 Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
367 reverted to the 7-day week. With the 6-day week the usual days
368 off were the 6th, 12th, 18th, 24th and 30th of the month.
369 (Source: Evitiar Zerubavel, _The Seven Day Circle_)
370
371
372 Mark Brader reported a similar story in "The Book of Calendars", edited
373 by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But:
374
375 From: Petteri Sulonen (via Usenet)
376 Date: 14 Jan 1999 00:00:00 GMT
377 Message-ID: <Petteri.Sulonen-1401991626030001@lapin-kulta.in.helsinki.fi>
378
379 If your source is correct, how come documents between 1929 -- 1940 were
380 still dated using the conventional, Gregorian calendar?
381
382 I can post a scan of a document dated December 1, 1934, signed by
383 Yenukidze, the secretary, on behalf of Kalinin, the President of the
384 Executive Committee of the Supreme Soviet, if you like.
385
386
387
388 Sweden (and Finland)
389
390 From: msb@sq.com (Mark Brader)
391 <a href="news:1996Jul6.012937.29190@sq.com">
392 Subject: Re: Gregorian reform -- a part of locale?
393 </a>
394 Date: 1996-07-06
395
396 In 1700, Denmark made the transition from Julian to Gregorian. Sweden
397 decided to *start* a transition in 1700 as well, but rather than have one of
398 those unsightly calendar gaps :-), they simply decreed that the next leap
399 year after 1696 would be in 1744 -- putting the whole country on a calendar
400 different from both Julian and Gregorian for a period of 40 years.
401
402 However, in 1704 something went wrong and the plan was not carried through;
403 they did, after all, have a leap year that year. And one in 1708. In 1712
404 they gave it up and went back to Julian, putting 30 days in February that
405 year!...
406
407 Then in 1753, Sweden made the transition to Gregorian in the usual manner,
408 getting there only 13 years behind the original schedule.
409
410 (A previous posting of this story was challenged, and Swedish readers
411 produced the following references to support it: "Tiderakning och historia"
412 by Natanael Beckman (1924) and "Tid, en bok om tiderakning och
413 kalendervasen" by Lars-Olof Lode'n (no date was given).)
414
415
416 Grotefend's data
417
418 From: "Michael Palmer" <mpalmer@netcom.com> [with one obvious typo fixed]
419 Subject: Re: Gregorian Calendar (was Re: Another FHC related question
420 Newsgroups: soc.genealogy.german
421 Date: Tue, 9 Feb 1999 02:32:48 -800
422 Message-ID: <199902091032.CAA09644@netcom10.netcom.com>
423
424 The following is a(n incomplete) listing, arranged chronologically, of
425 European states, with the date they converted from the Julian to the
426 Gregorian calendar:
427
428 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
429 Catholics and Danzig only)
430 09/20 Dec 1582 - France, Lorraine
431
432 21 Dec 1582/
433 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
434 10/21 Feb 1583 - bishopric of Liege (L"uttich)
435 13/24 Feb 1583 - bishopric of Augsburg
436 04/15 Oct 1583 - electorate of Trier
437 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
438 Salzburg, Brixen
439 13/24 Oct 1583 - Austrian Oberelsass and Breisgau
440 20/31 Oct 1583 - bishopric of Basel
441 02/13 Nov 1583 - duchy of J"ulich-Berg
442 02/13 Nov 1583 - electorate and city of K"oln
443 04/15 Nov 1583 - bishopric of W"urzburg
444 11/22 Nov 1583 - electorate of Mainz
445 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
446 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve
447 14/25 Dec 1583 - Steiermark
448
449 06/17 Jan 1584 - Austria and Bohemia
450 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn
451 12/23 Jan 1584 - Silesia and the Lausitz
452 22 Jan/
453 02 Feb 1584 - Hungary (legally on 21 Oct 1587)
454 Jun 1584 - Unterwalden
455 01/12 Jul 1584 - duchy of Westfalen
456
457 16/27 Jun 1585 - bishopric of Paderborn
458
459 14/25 Dec 1590 - Transylvania
460
461 22 Aug/
462 02 Sep 1612 - duchy of Prussia
463
464 13/24 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
473 15/26 Mar 1631 - bishopric of Hildesheim
474
475 1655 - Kanton Wallis
476
477 05/16 Feb 1682 - city of Strassburg
478
479 18 Feb/
480 01 Mar 1700 - Protestant Germany (including Swedish possessions in
481 Germany), Denmark, Norway
482 30 Jun/
483 12 Jul 1700 - Gelderland, Zutphen
484 10 Nov/
485 12 Dec 1700 - Utrecht, Overijssel
486
487 31 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
493 01 Jan 1750 - Pisa and Florence
494
495 02/14 Sep 1752 - Great Britain
496
497 17 Feb/
498 01 Mar 1753 - Sweden
499
500 1760-1812 - Graub"unden
501
502 The Russian empire (including Finland and the Baltic states) did not
503 convert to the Gregorian calendar until the Soviet revolution of 1917.
504
505 Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
506 Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
507 (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
508
509
510 ----- Time and time zones on Mars -----
511
512 Some people have adjusted their work schedules to fit Mars time.
513 Dozens of special Mars watches were built for Jet Propulsion
514 Laboratory workers who kept Mars time during the Mars Exploration
515 Rovers mission (2004). These timepieces look like normal Seikos and
516 Citizens but use Mars seconds rather than terrestrial seconds.
517
518 A Mars solar day is called a "sol" and has a mean period equal to
519 about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
520 divided into a conventional 24-hour clock, so each Mars second equals
521 about 1.02749125 terrestrial seconds.
522
523 The prime meridian of Mars goes through the center of the crater
524 Airy-0, named in honor of the British astronomer who built the
525 Greenwich telescope that defines Earth's prime meridian. Mean solar
526 time on the Mars prime meridian is called Mars Coordinated Time (MTC).
527
528 Each landed mission on Mars has adopted a different reference for
529 solar time keeping, so there is no real standard for Mars time zones.
530 For example, the Mars Exploration Rover project (2004) defined two
531 time zones "Local Solar Time A" and "Local Solar Time B" for its two
532 missions, each zone designed so that its time equals local true solar
533 time at approximately the middle of the nominal mission. Such a "time
534 zone" is not particularly suited for any application other than the
535 mission itself.
536
537 Many calendars have been proposed for Mars, but none have achieved
538 wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
539 sequential count of Mars solar days elapsed since about 1873-12-29
540 12:00 GMT.
541
542 The tz database does not currently support Mars time, but it is
543 documented here in the hopes that support will be added eventually.
544
545 Sources:
546
547 Michael Allison and Robert Schmunk,
548 "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
549 <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-03-15).
550
551 Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
552 (2004-01-14), pp A1, A20-A21.