1tzfile(5) File Formats Manual tzfile(5)
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6 tzfile - timezone information
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9 The timezone information files used by tzset(3) are typically found un‐
10 der a directory with a name like /usr/share/zoneinfo. These files use
11 the format described in Internet RFC 8536. Each file is a sequence of
12 8-bit bytes. In a file, a binary integer is represented by a sequence
13 of one or more bytes in network order (bigendian, or high-order byte
14 first), with all bits significant, a signed binary integer is repre‐
15 sented using two's complement, and a boolean is represented by a one-
16 byte binary integer that is either 0 (false) or 1 (true). The format
17 begins with a 44-byte header containing the following fields:
18 * The magic four-byte ASCII sequence “TZif” identifies the file as a
20 timezone information file.
21 * A byte identifying the version of the file's format (as of 2021, ei‐
23 ther an ASCII NUL, “2”, “3”, or “4”).
24 * Fifteen bytes containing zeros reserved for future use.
26 * Six four-byte integer values, in the following order:
28 tzh_ttisutcnt
30 The number of UT/local indicators stored in the file. (UT is
31 Universal Time.)
32 tzh_ttisstdcnt
34 The number of standard/wall indicators stored in the file.
35 tzh_leapcnt
37 The number of leap seconds for which data entries are stored
38 in the file.
39 tzh_timecnt
41 The number of transition times for which data entries are
42 stored in the file.
43 tzh_typecnt
45 The number of local time types for which data entries are
46 stored in the file (must not be zero).
47 tzh_charcnt
49 The number of bytes of time zone abbreviation strings stored
50 in the file.
51 The above header is followed by the following fields, whose lengths de‐
53 pend on the contents of the header:
54 * tzh_timecnt four-byte signed integer values sorted in ascending or‐
56 der. These values are written in network byte order. Each is used
57 as a transition time (as returned by time(2)) at which the rules for
58 computing local time change.
59 * tzh_timecnt one-byte unsigned integer values; each one but the last
61 tells which of the different types of local time types described in
62 the file is associated with the time period starting with the same-
63 indexed transition time and continuing up to but not including the
64 next transition time. (The last time type is present only for con‐
65 sistency checking with the POSIX-style TZ string described below.)
66 These values serve as indices into the next field.
67 * tzh_typecnt ttinfo entries, each defined as follows:
69 struct ttinfo {
71 int32_t tt_utoff;
72 unsigned char tt_isdst;
73 unsigned char tt_desigidx;
74 };
75 Each structure is written as a four-byte signed integer value for
77 tt_utoff, in network byte order, followed by a one-byte boolean for
78 tt_isdst and a one-byte value for tt_desigidx. In each structure,
79 tt_utoff gives the number of seconds to be added to UT, tt_isdst
80 tells whether tm_isdst should be set by localtime(3) and tt_desigidx
81 serves as an index into the array of time zone abbreviation bytes
82 that follow the ttinfo entries in the file; if the designated string
83 is "-00", the ttinfo entry is a placeholder indicating that local
84 time is unspecified. The tt_utoff value is never equal to -2**31, to
85 let 32-bit clients negate it without overflow. Also, in realistic
86 applications tt_utoff is in the range [-89999, 93599] (i.e., more
87 than -25 hours and less than 26 hours); this allows easy support by
88 implementations that already support the POSIX-required range
89 [-24:59:59, 25:59:59].
90 * tzh_charcnt bytes that represent time zone designations, which are
92 null-terminated byte strings, each indexed by the tt_desigidx values
93 mentioned above. The byte strings can overlap if one is a suffix of
94 the other. The encoding of these strings is not specified.
95 * tzh_leapcnt pairs of four-byte values, written in network byte order;
97 the first value of each pair gives the nonnegative time (as returned
98 by time(2)) at which a leap second occurs or at which the leap second
99 table expires; the second is a signed integer specifying the correc‐
100 tion, which is the total number of leap seconds to be applied during
101 the time period starting at the given time. The pairs of values are
102 sorted in strictly ascending order by time. Each pair denotes one
103 leap second, either positive or negative, except that if the last
104 pair has the same correction as the previous one, the last pair de‐
105 notes the leap second table's expiration time. Each leap second is
106 at the end of a UTC calendar month. The first leap second has a non‐
107 negative occurrence time, and is a positive leap second if and only
108 if its correction is positive; the correction for each leap second
109 after the first differs from the previous leap second by either 1 for
110 a positive leap second, or -1 for a negative leap second. If the
111 leap second table is empty, the leap-second correction is zero for
112 all timestamps; otherwise, for timestamps before the first occurrence
113 time, the leap-second correction is zero if the first pair's correc‐
114 tion is 1 or -1, and is unspecified otherwise (which can happen only
115 in files truncated at the start).
116 * tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte
118 boolean; they tell whether the transition times associated with local
119 time types were specified as standard time or local (wall clock)
120 time.
121 * tzh_ttisutcnt UT/local indicators, each stored as a one-byte boolean;
123 they tell whether the transition times associated with local time
124 types were specified as UT or local time. If a UT/local indicator is
125 set, the corresponding standard/wall indicator must also be set.
126 The standard/wall and UT/local indicators were designed for transform‐
128 ing a TZif file's transition times into transitions appropriate for an‐
129 other time zone specified via a POSIX-style TZ string that lacks rules.
130 For example, when TZ="EET-2EEST" and there is no TZif file "EET-2EEST",
131 the idea was to adapt the transition times from a TZif file with the
132 well-known name "posixrules" that is present only for this purpose and
133 is a copy of the file "Europe/Brussels", a file with a different UT
134 offset. POSIX does not specify this obsolete transformational behav‐
135 ior, the default rules are installation-dependent, and no implementa‐
136 tion is known to support this feature for timestamps past 2037, so
137 users desiring (say) Greek time should instead specify TZ="Eu‐
138 rope/Athens" for better historical coverage, falling back on
139 TZ="EET-2EEST,M3.5.0/3,M10.5.0/4" if POSIX conformance is required and
140 older timestamps need not be handled accurately.
141 The localtime(3) function normally uses the first ttinfo structure in
143 the file if either tzh_timecnt is zero or the time argument is less
144 than the first transition time recorded in the file.
145 Version 2 format
147 For version-2-format timezone files, the above header and data are fol‐
148 lowed by a second header and data, identical in format except that
149 eight bytes are used for each transition time or leap second time.
150 (Leap second counts remain four bytes.) After the second header and
151 data comes a newline-enclosed, POSIX-TZ-environment-variable-style
152 string for use in handling instants after the last transition time
153 stored in the file or for all instants if the file has no transitions.
154 The POSIX-style TZ string is empty (i.e., nothing between the newlines)
155 if there is no POSIX-style representation for such instants. If
156 nonempty, the POSIX-style TZ string must agree with the local time type
157 after the last transition time if present in the eight-byte data; for
158 example, given the string “WET0WEST,M3.5.0/1,M10.5.0” then if a last
159 transition time is in July, the transition's local time type must spec‐
160 ify a daylight-saving time abbreviated “WEST” that is one hour east of
161 UT. Also, if there is at least one transition, time type 0 is associ‐
162 ated with the time period from the indefinite past up to but not in‐
163 cluding the earliest transition time.
164 Version 3 format
166 For version-3-format timezone files, the POSIX-TZ-style string may use
167 two minor extensions to the POSIX TZ format, as described in
168 newtzset(3). First, the hours part of its transition times may be
169 signed and range from -167 through 167 instead of the POSIX-required
170 unsigned values from 0 through 24. Second, DST is in effect all year
171 if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the
172 difference between daylight saving and standard time.
173 Version 4 format
175 For version-4-format TZif files, the first leap second record can have
176 a correction that is neither +1 nor -1, to represent truncation of the
177 TZif file at the start. Also, if two or more leap second transitions
178 are present and the last entry's correction equals the previous one,
179 the last entry denotes the expiration of the leap second table instead
180 of a leap second; timestamps after this expiration are unreliable in
181 that future releases will likely add leap second entries after the ex‐
182 piration, and the added leap seconds will change how post-expiration
183 timestamps are treated.
184 Interoperability considerations
186 Future changes to the format may append more data.
187 Version 1 files are considered a legacy format and should not be gener‐
189 ated, as they do not support transition times after the year 2038.
190 Readers that understand only Version 1 must ignore any data that ex‐
191 tends beyond the calculated end of the version 1 data block.
192 Other than version 1, writers should generate the lowest version number
194 needed by a file's data. For example, a writer should generate a ver‐
195 sion 4 file only if its leap second table either expires or is trun‐
196 cated at the start. Likewise, a writer not generating a version 4 file
197 should generate a version 3 file only if TZ string extensions are nec‐
198 essary to accurately model transition times.
199 The sequence of time changes defined by the version 1 header and data
201 block should be a contiguous sub-sequence of the time changes defined
202 by the version 2+ header and data block, and by the footer. This
203 guideline helps obsolescent version 1 readers agree with current read‐
204 ers about timestamps within the contiguous sub-sequence. It also lets
205 writers not supporting obsolescent readers use a tzh_timecnt of zero in
206 the version 1 data block to save space.
207 When a TZif file contains a leap second table expiration time, TZif
209 readers should either refuse to process post-expiration timestamps, or
210 process them as if the expiration time did not exist (possibly with an
211 error indication).
212 Time zone designations should consist of at least three (3) and no more
214 than six (6) ASCII characters from the set of alphanumerics, “-”, and
215 “+”. This is for compatibility with POSIX requirements for time zone
216 abbreviations.
217 When reading a version 2 or higher file, readers should ignore the ver‐
219 sion 1 header and data block except for the purpose of skipping over
220 them.
221 Readers should calculate the total lengths of the headers and data
223 blocks and check that they all fit within the actual file size, as part
224 of a validity check for the file.
225 When a positive leap second occurs, readers should append an extra sec‐
227 ond to the local minute containing the second just before the leap sec‐
228 ond. If this occurs when the UTC offset is not a multiple of 60 sec‐
229 onds, the leap second occurs earlier than the last second of the local
230 minute and the minute's remaining local seconds are numbered through 60
231 instead of the usual 59; the UTC offset is unaffected.
232 Common interoperability issues
234 This section documents common problems in reading or writing TZif
235 files. Most of these are problems in generating TZif files for use by
236 older readers. The goals of this section are:
237 * to help TZif writers output files that avoid common pitfalls in older
239 or buggy TZif readers,
240 * to help TZif readers avoid common pitfalls when reading files gener‐
242 ated by future TZif writers, and
243 * to help any future specification authors see what sort of problems
245 arise when the TZif format is changed.
246 When new versions of the TZif format have been defined, a design goal
248 has been that a reader can successfully use a TZif file even if the
249 file is of a later TZif version than what the reader was designed for.
250 When complete compatibility was not achieved, an attempt was made to
251 limit glitches to rarely used timestamps and allow simple partial work‐
252 arounds in writers designed to generate new-version data useful even
253 for older-version readers. This section attempts to document these
254 compatibility issues and workarounds, as well as to document other com‐
255 mon bugs in readers.
256 Interoperability problems with TZif include the following:
258 * Some readers examine only version 1 data. As a partial workaround, a
260 writer can output as much version 1 data as possible. However, a
261 reader should ignore version 1 data, and should use version 2+ data
262 even if the reader's native timestamps have only 32 bits.
263 * Some readers designed for version 2 might mishandle timestamps after
265 a version 3 or higher file's last transition, because they cannot
266 parse extensions to POSIX in the TZ-like string. As a partial work‐
267 around, a writer can output more transitions than necessary, so that
268 only far-future timestamps are mishandled by version 2 readers.
269 * Some readers designed for version 2 do not support permanent daylight
271 saving time with transitions after 24:00 – e.g., a TZ string
272 “EST5EDT,0/0,J365/25” denoting permanent Eastern Daylight Time (-04).
273 As a workaround, a writer can substitute standard time for two time
274 zones east, e.g., “XXX3EDT4,0/0,J365/23” for a time zone with a
275 never-used standard time (XXX, -03) and negative daylight saving time
276 (EDT, -04) all year. Alternatively, as a partial workaround a writer
277 can substitute standard time for the next time zone east – e.g.,
278 “AST4” for permanent Atlantic Standard Time (-04).
279 * Some readers designed for version 2 or 3, and that require strict
281 conformance to RFC 8536, reject version 4 files whose leap second ta‐
282 bles are truncated at the start or that end in expiration times.
283 * Some readers ignore the footer, and instead predict future timestamps
285 from the time type of the last transition. As a partial workaround,
286 a writer can output more transitions than necessary.
287 * Some readers do not use time type 0 for timestamps before the first
289 transition, in that they infer a time type using a heuristic that
290 does not always select time type 0. As a partial workaround, a
291 writer can output a dummy (no-op) first transition at an early time.
292 * Some readers mishandle timestamps before the first transition that
294 has a timestamp not less than -2**31. Readers that support only
295 32-bit timestamps are likely to be more prone to this problem, for
296 example, when they process 64-bit transitions only some of which are
297 representable in 32 bits. As a partial workaround, a writer can out‐
298 put a dummy transition at timestamp -2**31.
299 * Some readers mishandle a transition if its timestamp has the minimum
301 possible signed 64-bit value. Timestamps less than -2**59 are not
302 recommended.
303 * Some readers mishandle POSIX-style TZ strings that contain “<” or
305 “>”. As a partial workaround, a writer can avoid using “<” or “>”
306 for time zone abbreviations containing only alphabetic characters.
307 * Many readers mishandle time zone abbreviations that contain non-ASCII
309 characters. These characters are not recommended.
310 * Some readers may mishandle time zone abbreviations that contain fewer
312 than 3 or more than 6 characters, or that contain ASCII characters
313 other than alphanumerics, “-”, and “+”. These abbreviations are not
314 recommended.
315 * Some readers mishandle TZif files that specify daylight-saving time
317 UT offsets that are less than the UT offsets for the corresponding
318 standard time. These readers do not support locations like Ireland,
319 which uses the equivalent of the POSIX TZ string
320 “IST-1GMT0,M10.5.0,M3.5.0/1”, observing standard time (IST, +01) in
321 summer and daylight saving time (GMT, +00) in winter. As a partial
322 workaround, a writer can output data for the equivalent of the POSIX
323 TZ string “GMT0IST,M3.5.0/1,M10.5.0”, thus swapping standard and day‐
324 light saving time. Although this workaround misidentifies which part
325 of the year uses daylight saving time, it records UT offsets and time
326 zone abbreviations correctly.
327 * Some readers generate ambiguous timestamps for positive leap seconds
329 that occur when the UTC offset is not a multiple of 60 seconds. For
330 example, in a timezone with UTC offset +01:23:45 and with a positive
331 leap second 78796801 (1972-06-30 23:59:60 UTC), some readers will map
332 both 78796800 and 78796801 to 01:23:45 local time the next day in‐
333 stead of mapping the latter to 01:23:46, and they will map 78796815
334 to 01:23:59 instead of to 01:23:60. This has not yet been a practi‐
335 cal problem, since no civil authority has observed such UTC offsets
336 since leap seconds were introduced in 1972.
337 Some interoperability problems are reader bugs that are listed here
339 mostly as warnings to developers of readers.
340 * Some readers do not support negative timestamps. Developers of dis‐
342 tributed applications should keep this in mind if they need to deal
343 with pre-1970 data.
344 * Some readers mishandle timestamps before the first transition that
346 has a nonnegative timestamp. Readers that do not support negative
347 timestamps are likely to be more prone to this problem.
348 * Some readers mishandle time zone abbreviations like “-08” that con‐
350 tain “+”, “-”, or digits.
351 * Some readers mishandle UT offsets that are out of the traditional
353 range of -12 through +12 hours, and so do not support locations like
354 Kiritimati that are outside this range.
355 * Some readers mishandle UT offsets in the range [-3599, -1] seconds
357 from UT, because they integer-divide the offset by 3600 to get 0 and
358 then display the hour part as “+00”.
359 * Some readers mishandle UT offsets that are not a multiple of one
361 hour, or of 15 minutes, or of 1 minute.
362
364 time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8).
365 Olson A, Eggert P, Murchison K. The Time Zone Information Format
367 (TZif). 2019 Feb. Internet RFC 8536 ⟨https://datatracker.ietf.org/
368 doc/html/rfc8536⟩ doi:10.17487/RFC8536 ⟨https://doi.org/10.17487/
369 RFC8536⟩.
370Time Zone Database tzfile(5)