6 Time and date functions
7 Names of time zone regions
8 Time zone abbreviations
11 ----- Time and date functions -----
13 These time and date functions are upwards compatible with POSIX.1,
14 an international standard for Unix-like systems.
15 As of this writing, the current edition of POSIX.1 is:
17 Information technology --Portable Operating System Interface (POSIX (R))
18 -- Part 1: System Application Program Interface (API) [C Language]
20 ANSI/IEEE Std 1003.1, 1996 Edition
23 POSIX.1 has the following properties and limitations.
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.
32 The POSIX.1 TZ string takes the following form:
34 stdoffset[dst[offset],date[/time],date[/time]]
39 are 3 or more characters specifying the standard
40 and daylight saving time (DST) zone names.
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.
50 takes the form `hh:[mm[:ss]]' and defaults to 02:00.
52 takes one of the following forms:
54 origin-1 day number not counting February 29
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).
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.
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 UTC" 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.)
79 * POSIX.1 requires that systems ignore leap seconds.
81 These are the extensions that have been made to the POSIX.1 functions:
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.
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
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.
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.")
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).
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.)
132 * These functions can account for leap seconds, thanks to Bradley White
135 Points of interest to folks with other systems:
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.
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.
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.
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 UTC.
161 This package takes care to do these conversions correctly.
163 The functions that are conditionally compiled if STD_INSPIRED is defined
164 should, at this point, be looked on primarily as food for thought. They are
165 not in any sense "standard compatible"--some are not, in fact, specified in
166 *any* standard. They do, however, represent responses of various authors to
167 standardization proposals.
169 Other time conversion proposals, in particular the one developed by folks at
170 Hewlett Packard, offer a wider selection of functions that provide capabilities
171 beyond those provided here. The absence of such functions from this package
172 is not meant to discourage the development, standardization, or use of such
173 functions. Rather, their absence reflects the decision to make this package
174 contain valid extensions to POSIX.1, to ensure its broad
175 acceptability. If more powerful time conversion functions can be standardized,
179 ----- Names of time zone rule files -----
181 The names of this package's installed time zone rule files are chosen to
182 help minimize possible future incompatibilities due to political events.
183 Ordinarily, names of countries are not used, to avoid incompatibilities
184 when countries change their name (e.g. Zaire->Congo) or
185 when locations change countries (e.g. Hong Kong from UK colony to China).
187 Names normally have the form AREA/LOCATION, where AREA is the name
188 of a continent or ocean, and LOCATION is the name of a specific
189 location within that region. North and South America share the same
190 area, `America'. Typical names are `Africa/Cairo', `America/New_York',
191 and `Pacific/Honolulu'.
193 Here are the general rules used for choosing location names,
194 in decreasing order of importance:
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'
229 The file `zone.tab' lists the geographical locations used to name
230 time zone rule files.
232 Older versions of this package used a different naming scheme,
233 and these older names are still supported.
234 See the file `backwards' for most of these older names
235 (e.g. `US/Eastern' instead of `America/New_York').
236 The other old-fashioned names still supported are
237 `WET', `CET', `MET', `EET' (see the file `europe'),
238 and `Factory' (see the file `factory').
241 ----- Time zone abbreviations -----
243 When this package is installed, it generates time zone abbreviations
244 like `EST' to be compatible with human tradition and POSIX.1.
245 Here are the general rules used for choosing time zone abbreviations,
246 in decreasing order of importance:
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
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:
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.
281 Application writers should note that these abbreviations are ambiguous
282 in practice: e.g. `EST' has a different meaning in Australia than
283 it does in the United States. In new applications, it's often better
284 to use numeric UTC offsets like `-0500' instead of time zone
285 abbreviations like `EST'; this avoids the ambiguity.