X-Git-Url: https://git.proxmox.com/?p=mirror_edk2.git;a=blobdiff_plain;f=StdLib%2FLibC%2FTime%2FTheory.txt;fp=StdLib%2FLibC%2FTime%2FTheory.txt;h=0000000000000000000000000000000000000000;hp=f8e77adbdfc2f2d005242578c8425d960aa935fe;hb=964f432b9b0afe103c41c7613fade3e699118afe;hpb=e2d3a25f1a3135221a9c8061e1b8f90245d727eb;ds=sidebyside diff --git a/StdLib/LibC/Time/Theory.txt b/StdLib/LibC/Time/Theory.txt deleted file mode 100644 index f8e77adbdf..0000000000 --- a/StdLib/LibC/Time/Theory.txt +++ /dev/null @@ -1,553 +0,0 @@ -# $NetBSD: Theory,v 1.8 2004/05/27 20:39:49 kleink Exp $ -@(#)Theory 7.15 - - ------ Outline ----- - - Time and date functions - Names of time zone regions - Time zone abbreviations - Calendrical issues - Time and time zones on Mars - - ------ Time and date functions ----- - -These time and date functions are upwards compatible with POSIX.1, -an international standard for UNIX-like systems. -As of this writing, the current edition of POSIX.1 is: - - Information technology --Portable Operating System Interface (POSIX (R)) - -- Part 1: System Application Program Interface (API) [C Language] - ISO/IEC 9945-1:1996 - ANSI/IEEE Std 1003.1, 1996 Edition - 1996-07-12 - -POSIX.1 has the following properties and limitations. - -* In POSIX.1, time display in a process is controlled by the - environment variable TZ. Unfortunately, the POSIX.1 TZ string takes - a form that is hard to describe and is error-prone in practice. - Also, POSIX.1 TZ strings can't deal with other (for example, Israeli) - daylight saving time rules, or situations where more than two - time zone abbreviations are used in an area. - - The POSIX.1 TZ string takes the following form: - - stdoffset[dst[offset],date[/time],date[/time]] - - where: - - std and dst - are 3 or more characters specifying the standard - and daylight saving time (DST) zone names. - offset - is of the form `[-]hh:[mm[:ss]]' and specifies the - offset west of UTC. The default DST offset is one hour - ahead of standard time. - date[/time],date[/time] - specifies the beginning and end of DST. If this is absent, - the system supplies its own rules for DST, and these can - differ from year to year; typically US DST rules are used. - time - takes the form `hh:[mm[:ss]]' and defaults to 02:00. - date - takes one of the following forms: - Jn (1<=n<=365) - origin-1 day number not counting February 29 - n (0<=n<=365) - origin-0 day number counting February 29 if present - Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) - for the dth day of week n of month m of the year, - where week 1 is the first week in which day d appears, - and `5' stands for the last week in which day d appears - (which may be either the 4th or 5th week). - -* In POSIX.1, when a TZ value like "EST5EDT" is parsed, - typically the current US DST rules are used, - but this means that the US DST rules are compiled into each program - that does time conversion. This means that when US time conversion - rules change (as in the United States in 1987), all programs that - do time conversion must be recompiled to ensure proper results. - -* In POSIX.1, there's no tamper-proof way for a process to learn the - system's best idea of local wall clock. (This is important for - applications that an administrator wants used only at certain times-- - without regard to whether the user has fiddled the "TZ" environment - variable. While an administrator can "do everything in UTC" to get - around the problem, doing so is inconvenient and precludes handling - daylight saving time shifts--as might be required to limit phone - calls to off-peak hours.) - -* POSIX.1 requires that systems ignore leap seconds. - -These are the extensions that have been made to the POSIX.1 functions: - -* The "TZ" environment variable is used in generating the name of a file - from which time zone information is read (or is interpreted a la - POSIX); "TZ" is no longer constrained to be a three-letter time zone - name followed by a number of hours and an optional three-letter - daylight time zone name. The daylight saving time rules to be used - for a particular time zone are encoded in the time zone file; - the format of the file allows U.S., Australian, and other rules to be - encoded, and allows for situations where more than two time zone - abbreviations are used. - - It was recognized that allowing the "TZ" environment variable to - take on values such as "America/New_York" might cause "old" programs - (that expect "TZ" to have a certain form) to operate incorrectly; - consideration was given to using some other environment variable - (for example, "TIMEZONE") to hold the string used to generate the - time zone information file name. In the end, however, it was decided - to continue using "TZ": it is widely used for time zone purposes; - separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; - and systems where "new" forms of "TZ" might cause problems can simply - use TZ values such as "EST5EDT" which can be used both by - "new" programs (a la POSIX) and "old" programs (as zone names and - offsets). - -* To handle places where more than two time zone abbreviations are used, - the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] - (where "tmp" is the value the function returns) to the time zone - abbreviation to be used. This differs from POSIX.1, where the elements - of tzname are only changed as a result of calls to tzset. - -* Since the "TZ" environment variable can now be used to control time - conversion, the "daylight" and "timezone" variables are no longer - needed. (These variables are defined and set by "tzset"; however, their - values will not be used by "localtime.") - -* The "localtime" function has been set up to deliver correct results - for near-minimum or near-maximum time_t values. (A comment in the - source code tells how to get compatibly wrong results). - -* A function "tzsetwall" has been added to arrange for the system's - best approximation to local wall clock time to be delivered by - subsequent calls to "localtime." Source code for portable - applications that "must" run on local wall clock time should call - "tzsetwall();" if such code is moved to "old" systems that don't - provide tzsetwall, you won't be able to generate an executable program. - (These time zone functions also arrange for local wall clock time to be - used if tzset is called--directly or indirectly--and there's no "TZ" - environment variable; portable applications should not, however, rely - on this behavior since it's not the way SVR2 systems behave.) - -* These functions can account for leap seconds, thanks to Bradley White - (bww@k.cs.cmu.edu). - -Points of interest to folks with other systems: - -* This package is already part of many POSIX-compliant hosts, - including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. - On such hosts, the primary use of this package - is to update obsolete time zone rule tables. - To do this, you may need to compile the time zone compiler - `zic' supplied with this package instead of using the system `zic', - since the format of zic's input changed slightly in late 1994, - and many vendors still do not support the new input format. - -* The UNIX Version 7 "timezone" function is not present in this package; - it's impossible to reliably map timezone's arguments (a "minutes west - of GMT" value and a "daylight saving time in effect" flag) to a - time zone abbreviation, and we refuse to guess. - Programs that in the past used the timezone function may now examine - tzname[localtime(&clock)->tm_isdst] to learn the correct time - zone abbreviation to use. Alternatively, use - localtime(&clock)->tm_zone if this has been enabled. - -* The 4.2BSD gettimeofday function is not used in this package. - This formerly let users obtain the current UTC offset and DST flag, - but this functionality was removed in later versions of BSD. - -* In SVR2, time conversion fails for near-minimum or near-maximum - time_t values when doing conversions for places that don't use UTC. - This package takes care to do these conversions correctly. - -The functions that are conditionally compiled if STD_INSPIRED is defined -should, at this point, be looked on primarily as food for thought. They are -not in any sense "standard compatible"--some are not, in fact, specified in -*any* standard. They do, however, represent responses of various authors to -standardization proposals. - -Other time conversion proposals, in particular the one developed by folks at -Hewlett Packard, offer a wider selection of functions that provide capabilities -beyond those provided here. The absence of such functions from this package -is not meant to discourage the development, standardization, or use of such -functions. Rather, their absence reflects the decision to make this package -contain valid extensions to POSIX.1, to ensure its broad -acceptability. If more powerful time conversion functions can be standardized, -so much the better. - - ------ Names of time zone rule files ----- - -The time zone rule file naming conventions attempt to strike a balance -among the following goals: - - * Uniquely identify every national region where clocks have all - agreed since 1970. This is essential for the intended use: static - clocks keeping local civil time. - - * Indicate to humans as to where that region is. This simplifes use. - - * Be robust in the presence of political changes. This reduces the - number of updates and backward-compatibility hacks. For example, - names of countries are ordinarily not used, to avoid - incompatibilities when countries change their name - (e.g. Zaire->Congo) or when locations change countries - (e.g. Hong Kong from UK colony to China). - - * Be portable to a wide variety of implementations. - This promotes use of the technology. - - * Use a consistent naming convention over the entire world. - This simplifies both use and maintenance. - -This naming convention is not intended for use by inexperienced users -to select TZ values by themselves (though they can of course examine -and reuse existing settings). Distributors should provide -documentation and/or a simple selection interface that explains the -names; see the 'tzselect' program supplied with this distribution for -one example. - -Names normally have the form AREA/LOCATION, where AREA is the name -of a continent or ocean, and LOCATION is the name of a specific -location within that region. North and South America share the same -area, `America'. Typical names are `Africa/Cairo', `America/New_York', -and `Pacific/Honolulu'. - -Here are the general rules used for choosing location names, -in decreasing order of importance: - - Use only valid POSIX file name components (i.e., the parts of - names other than `/'). Within a file name component, - use only ASCII letters, `.', `-' and `_'. Do not use - digits, as that might create an ambiguity with POSIX - TZ strings. A file name component must not exceed 14 - characters or start with `-'. E.g., prefer `Brunei' - to `Bandar_Seri_Begawan'. - Include at least one location per time zone rule set per country. - One such location is enough. Use ISO 3166 (see the file - iso3166.tab) to help decide whether something is a country. - If all the clocks in a country's region have agreed since 1970, - don't bother to include more than one location - even if subregions' clocks disagreed before 1970. - Otherwise these tables would become annoyingly large. - If a name is ambiguous, use a less ambiguous alternative; - e.g. many cities are named San Jose and Georgetown, so - prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'. - Keep locations compact. Use cities or small islands, not countries - or regions, so that any future time zone changes do not split - locations into different time zones. E.g. prefer `Paris' - to `France', since France has had multiple time zones. - Use mainstream English spelling, e.g. prefer `Rome' to `Roma', and - prefer `Athens' to the true name (which uses Greek letters). - The POSIX file name restrictions encourage this rule. - Use the most populous among locations in a country's time zone, - e.g. prefer `Shanghai' to `Beijing'. Among locations with - similar populations, pick the best-known location, - e.g. prefer `Rome' to `Milan'. - Use the singular form, e.g. prefer `Canary' to `Canaries'. - Omit common suffixes like `_Islands' and `_City', unless that - would lead to ambiguity. E.g. prefer `Cayman' to - `Cayman_Islands' and `Guatemala' to `Guatemala_City', - but prefer `Mexico_City' to `Mexico' because the country - of Mexico has several time zones. - Use `_' to represent a space. - Omit `.' from abbreviations in names, e.g. prefer `St_Helena' - to `St._Helena'. - Do not change established names if they only marginally - violate the above rules. For example, don't change - the existing name `Rome' to `Milan' merely because - Milan's population has grown to be somewhat greater - than Rome's. - If a name is changed, put its old spelling in the `backward' file. - -The file `zone.tab' lists the geographical locations used to name -time zone rule files. - -Older versions of this package used a different naming scheme, -and these older names are still supported. -See the file `backward' for most of these older names -(e.g. `US/Eastern' instead of `America/New_York'). -The other old-fashioned names still supported are -`WET', `CET', `MET', `EET' (see the file `europe'), -and `Factory' (see the file `factory'). - - ------ Time zone abbreviations ----- - -When this package is installed, it generates time zone abbreviations -like `EST' to be compatible with human tradition and POSIX.1. -Here are the general rules used for choosing time zone abbreviations, -in decreasing order of importance: - - Use abbreviations that consist of three or more ASCII letters. - Previous editions of this database also used characters like - ' ' and '?', but these characters have a special meaning to - the shell and cause commands like - set `date` - to have unexpected effects. - Previous editions of this rule required upper-case letters, - but the Congressman who introduced Chamorro Standard Time - preferred "ChST", so the rule has been relaxed. - - This rule guarantees that all abbreviations could have - been specified by a POSIX.1 TZ string. POSIX.1 - requires at least three characters for an - abbreviation. POSIX.1-1996 says that an abbreviation - cannot start with ':', and cannot contain ',', '-', - '+', NUL, or a digit. Draft 7 of POSIX 1003.1-200x - changes this rule to say that an abbreviation can - contain only '-', '+', and alphanumeric characters in - the current locale. To be portable to both sets of - rules, an abbreviation must therefore use only ASCII - letters, as these are the only letters that are - alphabetic in all locales. - - Use abbreviations that are in common use among English-speakers, - e.g. `EST' for Eastern Standard Time in North America. - We assume that applications translate them to other languages - as part of the normal localization process; for example, - a French application might translate `EST' to `HNE'. - - For zones whose times are taken from a city's longitude, use the - traditional xMT notation, e.g. `PMT' for Paris Mean Time. - The only name like this in current use is `GMT'. - - If there is no common English abbreviation, abbreviate the English - translation of the usual phrase used by native speakers. - If this is not available or is a phrase mentioning the country - (e.g. ``Cape Verde Time''), then: - - When a country has a single or principal time zone region, - append `T' to the country's ISO code, e.g. `CVT' for - Cape Verde Time. For summer time append `ST'; - for double summer time append `DST'; etc. - When a country has multiple time zones, take the first three - letters of an English place name identifying each zone - and then append `T', `ST', etc. as before; - e.g. `VLAST' for VLAdivostok Summer Time. - - Use "zzz" for locations while uninhabited. The mnemonic is that - these locations are, in some sense, asleep. - -Application writers should note that these abbreviations are ambiguous -in practice: e.g. `EST' has a different meaning in Australia than -it does in the United States. In new applications, it's often better -to use numeric UTC offsets like `-0500' instead of time zone -abbreviations like `EST'; this avoids the ambiguity. - - ------ Calendrical issues ----- - -Calendrical issues are a bit out of scope for a time zone database, -but they indicate the sort of problems that we would run into if we -extended the time zone database further into the past. An excellent -resource in this area is Nachum Dershowitz and Edward M. Reingold, - -Calendrical Calculations -, Cambridge University Press (1997). Other information and -sources are given below. They sometimes disagree. - - -France - -Gregorian calendar adopted 1582-12-20. -French Revolutionary calendar used 1793-11-24 through 1805-12-31, -and (in Paris only) 1871-05-06 through 1871-05-23. - - -Russia - -From Chris Carrier <72157.3334@CompuServe.COM> (1996-12-02): -On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar'' -with 30-day months plus 5 holidays, with a 5-day week. -On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the -Gregorian calendar while retaining the 6-day week; on 1940-06-27 it -reverted to the 7-day week. With the 6-day week the usual days -off were the 6th, 12th, 18th, 24th and 30th of the month. -(Source: Evitiar Zerubavel, _The Seven Day Circle_) - - -Mark Brader reported a similar story in "The Book of Calendars", edited -by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: - -From: Petteri Sulonen (via Usenet) -Date: 14 Jan 1999 00:00:00 GMT -Message-ID: - -If your source is correct, how come documents between 1929 -- 1940 were -still dated using the conventional, Gregorian calendar? - -I can post a scan of a document dated December 1, 1934, signed by -Yenukidze, the secretary, on behalf of Kalinin, the President of the -Executive Committee of the Supreme Soviet, if you like. - - - -Sweden (and Finland) - -From: msb@sq.com (Mark Brader) - -Subject: Re: Gregorian reform -- a part of locale? - -Date: 1996-07-06 - -In 1700, Denmark made the transition from Julian to Gregorian. Sweden -decided to *start* a transition in 1700 as well, but rather than have one of -those unsightly calendar gaps :-), they simply decreed that the next leap -year after 1696 would be in 1744 -- putting the whole country on a calendar -different from both Julian and Gregorian for a period of 40 years. - -However, in 1704 something went wrong and the plan was not carried through; -they did, after all, have a leap year that year. And one in 1708. In 1712 -they gave it up and went back to Julian, putting 30 days in February that -year!... - -Then in 1753, Sweden made the transition to Gregorian in the usual manner, -getting there only 13 years behind the original schedule. - -(A previous posting of this story was challenged, and Swedish readers -produced the following references to support it: "Tiderakning och historia" -by Natanael Beckman (1924) and "Tid, en bok om tiderakning och -kalendervasen" by Lars-Olof Lode'n (no date was given).) - - -Grotefend's data - -From: "Michael Palmer" [with one obvious typo fixed] -Subject: Re: Gregorian Calendar (was Re: Another FHC related question -Newsgroups: soc.genealogy.german -Date: Tue, 9 Feb 1999 02:32:48 -800 -Message-ID: <199902091032.CAA09644@netcom10.netcom.com> - -The following is a(n incomplete) listing, arranged chronologically, of -European states, with the date they converted from the Julian to the -Gregorian calendar: - -04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman - Catholics and Danzig only) -09/20 Dec 1582 - France, Lorraine - -21 Dec 1582/ - 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau -10/21 Feb 1583 - bishopric of Liege (L"uttich) -13/24 Feb 1583 - bishopric of Augsburg -04/15 Oct 1583 - electorate of Trier -05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, - Salzburg, Brixen -13/24 Oct 1583 - Austrian Oberelsass and Breisgau -20/31 Oct 1583 - bishopric of Basel -02/13 Nov 1583 - duchy of J"ulich-Berg -02/13 Nov 1583 - electorate and city of K"oln -04/15 Nov 1583 - bishopric of W"urzburg -11/22 Nov 1583 - electorate of Mainz -16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden -17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve -14/25 Dec 1583 - Steiermark - -06/17 Jan 1584 - Austria and Bohemia -11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn -12/23 Jan 1584 - Silesia and the Lausitz -22 Jan/ - 02 Feb 1584 - Hungary (legally on 21 Oct 1587) - Jun 1584 - Unterwalden -01/12 Jul 1584 - duchy of Westfalen - -16/27 Jun 1585 - bishopric of Paderborn - -14/25 Dec 1590 - Transylvania - -22 Aug/ - 02 Sep 1612 - duchy of Prussia - -13/24 Dec 1614 - Pfalz-Neuburg - - 1617 - duchy of Kurland (reverted to the Julian calendar in - 1796) - - 1624 - bishopric of Osnabr"uck - - 1630 - bishopric of Minden - -15/26 Mar 1631 - bishopric of Hildesheim - - 1655 - Kanton Wallis - -05/16 Feb 1682 - city of Strassburg - -18 Feb/ - 01 Mar 1700 - Protestant Germany (including Swedish possessions in - Germany), Denmark, Norway -30 Jun/ - 12 Jul 1700 - Gelderland, Zutphen -10 Nov/ - 12 Dec 1700 - Utrecht, Overijssel - -31 Dec 1700/ - 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva, - Turgau, and Schaffhausen - - 1724 - Glarus, Appenzell, and the city of St. Gallen - -01 Jan 1750 - Pisa and Florence - -02/14 Sep 1752 - Great Britain - -17 Feb/ - 01 Mar 1753 - Sweden - -1760-1812 - Graub"unden - -The Russian empire (including Finland and the Baltic states) did not -convert to the Gregorian calendar until the Soviet revolution of 1917. - -Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen -Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend -(Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. - - ------ Time and time zones on Mars ----- - -Some people have adjusted their work schedules to fit Mars time. -Dozens of special Mars watches were built for Jet Propulsion -Laboratory workers who kept Mars time during the Mars Exploration -Rovers mission (2004). These timepieces look like normal Seikos and -Citizens but use Mars seconds rather than terrestrial seconds. - -A Mars solar day is called a "sol" and has a mean period equal to -about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is -divided into a conventional 24-hour clock, so each Mars second equals -about 1.02749125 terrestrial seconds. - -The prime meridian of Mars goes through the center of the crater -Airy-0, named in honor of the British astronomer who built the -Greenwich telescope that defines Earth's prime meridian. Mean solar -time on the Mars prime meridian is called Mars Coordinated Time (MTC). - -Each landed mission on Mars has adopted a different reference for -solar time keeping, so there is no real standard for Mars time zones. -For example, the Mars Exploration Rover project (2004) defined two -time zones "Local Solar Time A" and "Local Solar Time B" for its two -missions, each zone designed so that its time equals local true solar -time at approximately the middle of the nominal mission. Such a "time -zone" is not particularly suited for any application other than the -mission itself. - -Many calendars have been proposed for Mars, but none have achieved -wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a -sequential count of Mars solar days elapsed since about 1873-12-29 -12:00 GMT. - -The tz database does not currently support Mars time, but it is -documented here in the hopes that support will be added eventually. - -Sources: - -Michael Allison and Robert Schmunk, -"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" - (2004-03-15). - -Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times -(2004-01-14), pp A1, A20-A21.