1@(#)Theory 7.5 2 3 4----- Outline ----- 5 6 Time and date functions 7 Names of time zone regions 8 Time zone abbreviations 9 10 11----- Time and date functions ----- 12 13These time and date functions are upwards compatible with POSIX.1, 14an international standard for Unix-like systems. 15As of this writing, the current edition of POSIX.1 is: 16 17 Information technology --Portable Operating System Interface (POSIX (R)) 18 -- Part 1: System Application Program Interface (API) [C Language] 19 ISO/IEC 9945-1:1996 20 ANSI/IEEE Std 1003.1, 1996 Edition 21 1996-07-12 22 23POSIX.1 has the following properties and limitations. 24 25* In POSIX.1, time display in a process is controlled by the 26 environment variable TZ. Unfortunately, the POSIX.1 TZ string takes 27 a form that is hard to describe and is error-prone in practice. 28 Also, POSIX.1 TZ strings can't deal with other (for example, Israeli) 29 daylight saving time rules, or situations where more than two 30 time zone abbreviations are used in an area. 31 32 The POSIX.1 TZ string takes the following form: 33 34 stdoffset[dst[offset],date[/time],date[/time]] 35 36 where: 37 38 std and dst 39 are 3 or more characters specifying the standard 40 and daylight saving time (DST) zone names. 41 offset 42 is of the form `[-]hh:[mm[:ss]]' and specifies the 43 offset west of UTC. The default DST offset is one hour 44 ahead of standard time. 45 date[/time],date[/time] 46 specifies the beginning and end of DST. If this is absent, 47 the system supplies its own rules for DST, and these can 48 differ from year to year; typically US DST rules are used. 49 time 50 takes the form `hh:[mm[:ss]]' and defaults to 02:00. 51 date 52 takes one of the following forms: 53 Jn (1<=n<=365) 54 origin-1 day number not counting February 29 55 n (0<=n<=365) 56 origin-0 day number counting February 29 if present 57 Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12) 58 for the dth day of week n of month m of the year, 59 where week 1 is the first week in which day d appears, 60 and `5' stands for the last week in which day d appears 61 (which may be either the 4th or 5th week). 62 63* In POSIX.1, when a TZ value like "EST5EDT" is parsed, 64 typically the current US DST rules are used, 65 but this means that the US DST rules are compiled into each program 66 that does time conversion. This means that when US time conversion 67 rules change (as in the United States in 1987), all programs that 68 do time conversion must be recompiled to ensure proper results. 69 70* In POSIX.1, there's no tamper-proof way for a process to learn the 71 system's best idea of local wall clock. (This is important for 72 applications that an administrator wants used only at certain times-- 73 without regard to whether the user has fiddled the "TZ" environment 74 variable. While an administrator can "do everything in GMT" to get 75 around the problem, doing so is inconvenient and precludes handling 76 daylight saving time shifts--as might be required to limit phone 77 calls to off-peak hours.) 78 79* POSIX.1 requires that systems ignore leap seconds. 80 81These are the extensions that have been made to the POSIX.1 functions: 82 83* The "TZ" environment variable is used in generating the name of a file 84 from which time zone information is read (or is interpreted a la 85 POSIX); "TZ" is no longer constrained to be a three-letter time zone 86 name followed by a number of hours and an optional three-letter 87 daylight time zone name. The daylight saving time rules to be used 88 for a particular time zone are encoded in the time zone file; 89 the format of the file allows U.S., Australian, and other rules to be 90 encoded, and allows for situations where more than two time zone 91 abbreviations are used. 92 93 It was recognized that allowing the "TZ" environment variable to 94 take on values such as "America/New_York" might cause "old" programs 95 (that expect "TZ" to have a certain form) to operate incorrectly; 96 consideration was given to using some other environment variable 97 (for example, "TIMEZONE") to hold the string used to generate the 98 time zone information file name. In the end, however, it was decided 99 to continue using "TZ": it is widely used for time zone purposes; 100 separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; 101 and systems where "new" forms of "TZ" might cause problems can simply 102 use TZ values such as "EST5EDT" which can be used both by 103 "new" programs (a la POSIX) and "old" programs (as zone names and 104 offsets). 105 106* To handle places where more than two time zone abbreviations are used, 107 the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] 108 (where "tmp" is the value the function returns) to the time zone 109 abbreviation to be used. This differs from POSIX.1, where the elements 110 of tzname are only changed as a result of calls to tzset. 111 112* Since the "TZ" environment variable can now be used to control time 113 conversion, the "daylight" and "timezone" variables are no longer 114 needed. (These variables are defined and set by "tzset"; however, their 115 values will not be used by "localtime.") 116 117* The "localtime" function has been set up to deliver correct results 118 for near-minimum or near-maximum time_t values. (A comment in the 119 source code tells how to get compatibly wrong results). 120 121* A function "tzsetwall" has been added to arrange for the system's 122 best approximation to local wall clock time to be delivered by 123 subsequent calls to "localtime." Source code for portable 124 applications that "must" run on local wall clock time should call 125 "tzsetwall();" if such code is moved to "old" systems that don't 126 provide tzsetwall, you won't be able to generate an executable program. 127 (These time zone functions also arrange for local wall clock time to be 128 used if tzset is called--directly or indirectly--and there's no "TZ" 129 environment variable; portable applications should not, however, rely 130 on this behavior since it's not the way SVR2 systems behave.) 131 132* These functions can account for leap seconds, thanks to Bradley White 133 (bww@k.cs.cmu.edu). 134 135Points of interest to folks with other systems: 136 137* This package is already part of many POSIX-compliant hosts, 138 including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 139 On such hosts, the primary use of this package 140 is to update obsolete time zone rule tables. 141 To do this, you may need to compile the time zone compiler 142 `zic' supplied with this package instead of using the system `zic', 143 since the format of zic's input changed slightly in late 1994, 144 and many vendors still do not support the new input format. 145 146* The Unix Version 7 "timezone" function is not present in this package; 147 it's impossible to reliably map timezone's arguments (a "minutes west 148 of GMT" value and a "daylight saving time in effect" flag) to a 149 time zone abbreviation, and we refuse to guess. 150 Programs that in the past used the timezone function may now examine 151 tzname[localtime(&clock)->tm_isdst] to learn the correct time 152 zone abbreviation to use. Alternatively, use 153 localtime(&clock)->tm_zone if this has been enabled. 154 155* The 4.2BSD gettimeofday function is not used in this package. 156 This formerly let users obtain the current UTC offset and DST flag, 157 but this functionality was removed in later versions of BSD. 158 159* In SVR2, time conversion fails for near-minimum or near-maximum 160 time_t values when doing conversions for places that don't use GMT. 161 This package takes care to do these conversions correctly. 162 163The functions that are conditionally compiled if STD_INSPIRED is defined 164should, at this point, be looked on primarily as food for thought. They are 165not in any sense "standard compatible"--some are not, in fact, specified in 166*any* standard. They do, however, represent responses of various authors to 167standardization proposals. 168 169Other time conversion proposals, in particular the one developed by folks at 170Hewlett Packard, offer a wider selection of functions that provide capabilities 171beyond those provided here. The absence of such functions from this package 172is not meant to discourage the development, standardization, or use of such 173functions. Rather, their absence reflects the decision to make this package 174contain valid extensions to POSIX.1, to ensure its broad 175acceptability. If more powerful time conversion functions can be standardized, 176so much the better. 177 178 179----- Names of time zone rule files ----- 180 181The names of this package's installed time zone rule files are chosen to 182help minimize possible future incompatibilities due to political events. 183Ordinarily, names of countries are not used, to avoid incompatibilities 184when countries change their name (e.g. Zaire->Congo) or 185when locations change countries (e.g. Hong Kong from UK colony to China). 186 187Names normally have the form AREA/LOCATION, where AREA is the name 188of a continent or ocean, and LOCATION is the name of a specific 189location within that region. North and South America share the same 190area, `America'. Typical names are `Africa/Cairo', `America/New_York', 191and `Pacific/Honolulu'. 192 193Here are the general rules used for choosing location names, 194in decreasing order of importance: 195 196 Use only valid Posix file names. Use only Ascii letters, digits, `.', 197 `-' and `_'. Do not exceed 14 characters or start with `-'. 198 E.g. prefer `Brunei' to `Bandar_Seri_Begawan'. 199 Include at least one location per time zone rule set per country. 200 One such location is enough. 201 If all the clocks in a country's region have agreed since 1970, 202 don't bother to include more than one location 203 even if subregions' clocks disagreed before 1970. 204 Otherwise these tables would become annoyingly large. 205 If a name is ambiguous, use a less ambiguous alternative; 206 e.g. many cities are named San Jose and Georgetown, so 207 prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'. 208 Keep locations compact. Use cities or small islands, not countries 209 or regions, so that any future time zone changes do not split 210 locations into different time zones. E.g. prefer `Paris' 211 to `France', since France has had multiple time zones. 212 Use traditional English spelling, e.g. prefer `Rome' to `Roma', and 213 prefer `Athens' to the true name (which uses Greek letters). 214 The Posix file name restrictions encourage this rule. 215 Use the most populous among locations in a country's time zone, 216 e.g. prefer `Shanghai' to `Beijing'. Among locations with 217 similar populations, pick the best-known location, 218 e.g. prefer `Rome' to `Milan'. 219 Use the singular form, e.g. prefer `Canary' to `Canaries'. 220 Omit common suffixes like `_Islands' and `_City', unless that 221 would lead to ambiguity. E.g. prefer `Cayman' to 222 `Cayman_Islands' and `Guatemala' to `Guatemala_City', 223 but prefer `Mexico_City' to `Mexico' because the country 224 of Mexico has several time zones. 225 Use `_' to represent a space. 226 Omit `.' from abbreviations in names, e.g. prefer `St_Helena' 227 to `St._Helena'. 228 229The file `zone.tab' lists the geographical locations used to name 230time zone rule files. 231 232Older versions of this package used a different naming scheme, 233and these older names are still supported. 234See the file `backwards' for most of these older names 235(e.g. `US/Eastern' instead of `America/New_York'). 236The other old-fashioned names still supported are 237`WET', `CET', `MET', `EET' (see the file `europe'), 238and `Factory' (see the file `factory'). 239 240 241----- Time zone abbreviations ----- 242 243When this package is installed, it generates time zone abbreviations 244like `EST' to be compatible with human tradition and POSIX.1. 245Here are the general rules used for choosing time zone abbreviations, 246in decreasing order of importance: 247 248 Use abbreviations that consist of 3 or more upper-case Ascii letters, 249 except use "___" for locations while uninhabited. 250 Posix.1 requires at least 3 characters, and the restriction to 251 upper-case Ascii letters follows most traditions. 252 Previous editions of this database also used characters like 253 ' ' and '?', but these characters have a special meaning to 254 the shell and cause commands like 255 set `date` 256 to have unexpected effects. In theory, the character set could 257 be !%./@A-Z^_a-z{}, but these tables use only upper-case 258 Ascii letters (and "___"). 259 Use abbreviations that are in common use among English-speakers, 260 e.g. `EST' for Eastern Standard Time in North America. 261 We assume that applications translate them to other languages 262 as part of the normal localization process; for example, 263 a French application might translate `EST' to `HNE'. 264 For zones whose times are taken from a city's longitude, use the 265 traditional xMT notation, e.g. `PMT' for Paris Mean Time. 266 The only name like this in current use is `GMT'. 267 If there is no common English abbreviation, abbreviate the English 268 translation of the usual phrase used by native speakers. 269 If this is not available or is a phrase mentioning the country 270 (e.g. ``Cape Verde Time''), then: 271 272 When a country has a single or principal time zone region, 273 append `T' to the country's ISO code, e.g. `CVT' for 274 Cape Verde Time. For summer time append `ST'; 275 for double summer time append `DST'; etc. 276 When a country has multiple time zones, take the first three 277 letters of an English place name identifying each zone 278 and then append `T', `ST', etc. as before; 279 e.g. `VLAST' for VLAdivostok Summer Time. 280 281Application writers should note that these abbreviations are ambiguous 282in practice: e.g. `EST' has a different meaning in Australia than 283it does in the United States. In new applications, it's often better 284to use numeric GMT offsets like `-0500' instead of time zone 285abbreviations like `EST'; this avoids the ambiguity. 286