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diff --git a/man5/tzfile.5 b/man5/tzfile.5 new file mode 100644 index 0000000..59d9f6b --- /dev/null +++ b/man5/tzfile.5 @@ -0,0 +1,496 @@ +.\" This file is in the public domain, so clarified as of +.\" 1996-06-05 by Arthur David Olson. +.TH tzfile 5 "" "Time Zone Database" +.SH NAME +tzfile \- timezone information +.SH DESCRIPTION +.ie '\(lq'' .ds lq \&"\" +.el .ds lq \(lq\" +.ie '\(rq'' .ds rq \&"\" +.el .ds rq \(rq\" +.de q +\\$3\*(lq\\$1\*(rq\\$2 +.. +.ie \n(.g .ds - \f(CR-\fP +.el .ds - \- +The timezone information files used by +.BR tzset (3) +are typically found under a directory with a name like +.IR /usr/share/zoneinfo . +These files use the format described in Internet RFC 8536. +Each file is a sequence of 8-bit bytes. +In a file, a binary integer is represented by a sequence of one or +more bytes in network order (bigendian, or high-order byte first), +with all bits significant, +a signed binary integer is represented using two's complement, +and a boolean is represented by a one-byte binary integer that is +either 0 (false) or 1 (true). +The format begins with a 44-byte header containing the following fields: +.IP * 2 +The magic four-byte ASCII sequence +.q "TZif" +identifies the file as a timezone information file. +.IP * +A byte identifying the version of the file's format +(as of 2021, either an ASCII NUL, +.q "2", +.q "3", +or +.q "4" ). +.IP * +Fifteen bytes containing zeros reserved for future use. +.IP * +Six four-byte integer values, in the following order: +.RS +.TP +.B tzh_ttisutcnt +The number of UT/local indicators stored in the file. +(UT is Universal Time.) +.TP +.B tzh_ttisstdcnt +The number of standard/wall indicators stored in the file. +.TP +.B tzh_leapcnt +The number of leap seconds for which data entries are stored in the file. +.TP +.B tzh_timecnt +The number of transition times for which data entries are stored +in the file. +.TP +.B tzh_typecnt +The number of local time types for which data entries are stored +in the file (must not be zero). +.TP +.B tzh_charcnt +The number of bytes of time zone abbreviation strings +stored in the file. +.RE +.PP +The above header is followed by the following fields, whose lengths +depend on the contents of the header: +.IP * 2 +.B tzh_timecnt +four-byte signed integer values sorted in ascending order. +These values are written in network byte order. +Each is used as a transition time (as returned by +.BR time (2)) +at which the rules for computing local time change. +.IP * +.B tzh_timecnt +one-byte unsigned integer values; +each one but the last tells which of the different types of local time types +described in the file is associated with the time period +starting with the same-indexed transition time +and continuing up to but not including the next transition time. +(The last time type is present only for consistency checking with the +POSIX-style TZ string described below.) +These values serve as indices into the next field. +.IP * +.B tzh_typecnt +.B ttinfo +entries, each defined as follows: +.in +.5i +.sp +.nf +.ta .5i +\w'unsigned char\0\0'u +struct ttinfo { + int32_t tt_utoff; + unsigned char tt_isdst; + unsigned char tt_desigidx; +}; +.in -.5i +.fi +.sp +Each structure is written as a four-byte signed integer value for +.BR tt_utoff , +in network byte order, followed by a one-byte boolean for +.B tt_isdst +and a one-byte value for +.BR tt_desigidx . +In each structure, +.B tt_utoff +gives the number of seconds to be added to UT, +.B tt_isdst +tells whether +.B tm_isdst +should be set by +.BR localtime (3) +and +.B tt_desigidx +serves as an index into the array of time zone abbreviation bytes +that follow the +.B ttinfo +entries in the file; if the designated string is "\*-00", the +.B ttinfo +entry is a placeholder indicating that local time is unspecified. +The +.B tt_utoff +value is never equal to \-2**31, to let 32-bit clients negate it without +overflow. +Also, in realistic applications +.B tt_utoff +is in the range [\-89999, 93599] (i.e., more than \-25 hours and less +than 26 hours); this allows easy support by implementations that +already support the POSIX-required range [\-24:59:59, 25:59:59]. +.IP * +.B tzh_charcnt +bytes that represent time zone designations, +which are null-terminated byte strings, each indexed by the +.B tt_desigidx +values mentioned above. +The byte strings can overlap if one is a suffix of the other. +The encoding of these strings is not specified. +.IP * +.B tzh_leapcnt +pairs of four-byte values, written in network byte order; +the first value of each pair gives the nonnegative time +(as returned by +.BR time (2)) +at which a leap second occurs or at which the leap second table expires; +the second is a signed integer specifying the correction, which is the +.I total +number of leap seconds to be applied during the time period +starting at the given time. +The pairs of values are sorted in strictly ascending order by time. +Each pair denotes one leap second, either positive or negative, +except that if the last pair has the same correction as the previous one, +the last pair denotes the leap second table's expiration time. +Each leap second is at the end of a UTC calendar month. +The first leap second has a nonnegative occurrence time, +and is a positive leap second if and only if its correction is positive; +the correction for each leap second after the first differs +from the previous leap second by either 1 for a positive leap second, +or \-1 for a negative leap second. +If the leap second table is empty, the leap-second correction is zero +for all timestamps; +otherwise, for timestamps before the first occurrence time, +the leap-second correction is zero if the first pair's correction is 1 or \-1, +and is unspecified otherwise (which can happen only in files +truncated at the start). +.IP * +.B tzh_ttisstdcnt +standard/wall indicators, each stored as a one-byte boolean; +they tell whether the transition times associated with local time types +were specified as standard time or local (wall clock) time. +.IP * +.B tzh_ttisutcnt +UT/local indicators, each stored as a one-byte boolean; +they tell whether the transition times associated with local time types +were specified as UT or local time. +If a UT/local indicator is set, the corresponding standard/wall indicator +must also be set. +.PP +The standard/wall and UT/local indicators were designed for +transforming a TZif file's transition times into transitions appropriate +for another time zone specified via a POSIX-style TZ string that lacks rules. +For example, when TZ="EET\*-2EEST" and there is no TZif file "EET\*-2EEST", +the idea was to adapt the transition times from a TZif file with the +well-known name "posixrules" that is present only for this purpose and +is a copy of the file "Europe/Brussels", a file with a different UT offset. +POSIX does not specify this obsolete transformational behavior, +the default rules are installation-dependent, and no implementation +is known to support this feature for timestamps past 2037, +so users desiring (say) Greek time should instead specify +TZ="Europe/Athens" for better historical coverage, falling back on +TZ="EET\*-2EEST,M3.5.0/3,M10.5.0/4" if POSIX conformance is required +and older timestamps need not be handled accurately. +.PP +The +.BR localtime (3) +function +normally uses the first +.B ttinfo +structure in the file +if either +.B tzh_timecnt +is zero or the time argument is less than the first transition time recorded +in the file. +.SS Version 2 format +For version-2-format timezone files, +the above header and data are followed by a second header and data, +identical in format except that +eight bytes are used for each transition time or leap second time. +(Leap second counts remain four bytes.) +After the second header and data comes a newline-enclosed, +POSIX-TZ-environment-variable-style string for use in handling instants +after the last transition time stored in the file +or for all instants if the file has no transitions. +The POSIX-style TZ string is empty (i.e., nothing between the newlines) +if there is no POSIX-style representation for such instants. +If nonempty, the POSIX-style TZ string must agree with the local time +type after the last transition time if present in the eight-byte data; +for example, given the string +.q "WET0WEST,M3.5.0/1,M10.5.0" +then if a last transition time is in July, the transition's local time +type must specify a daylight-saving time abbreviated +.q "WEST" +that is one hour east of UT. +Also, if there is at least one transition, time type 0 is associated +with the time period from the indefinite past up to but not including +the earliest transition time. +.SS Version 3 format +For version-3-format timezone files, the POSIX-TZ-style string may +use two minor extensions to the POSIX TZ format, as described in +.BR newtzset (3). +First, the hours part of its transition times may be signed and range from +\-167 through 167 instead of the POSIX-required unsigned values +from 0 through 24. +Second, DST is in effect all year if it starts +January 1 at 00:00 and ends December 31 at 24:00 plus the difference +between daylight saving and standard time. +.SS Version 4 format +For version-4-format TZif files, +the first leap second record can have a correction that is neither ++1 nor \-1, to represent truncation of the TZif file at the start. +Also, if two or more leap second transitions are present and the last +entry's correction equals the previous one, the last entry +denotes the expiration of the leap second table instead of a leap second; +timestamps after this expiration are unreliable in that future +releases will likely add leap second entries after the expiration, and +the added leap seconds will change how post-expiration timestamps are treated. +.SS Interoperability considerations +Future changes to the format may append more data. +.PP +Version 1 files are considered a legacy format and +should not be generated, as they do not support transition +times after the year 2038. +Readers that understand only Version 1 must ignore +any data that extends beyond the calculated end of the version +1 data block. +.PP +Other than version 1, writers should generate +the lowest version number needed by a file's data. +For example, a writer should generate a version 4 file +only if its leap second table either expires or is truncated at the start. +Likewise, a writer not generating a version 4 file +should generate a version 3 file only if +TZ string extensions are necessary to accurately +model transition times. +.PP +The sequence of time changes defined by the version 1 +header and data block should be a contiguous sub-sequence +of the time changes defined by the version 2+ header and data +block, and by the footer. +This guideline helps obsolescent version 1 readers +agree with current readers about timestamps within the +contiguous sub-sequence. It also lets writers not +supporting obsolescent readers use a +.B tzh_timecnt +of zero +in the version 1 data block to save space. +.PP +When a TZif file contains a leap second table expiration +time, TZif readers should either refuse to process +post-expiration timestamps, or process them as if the expiration +time did not exist (possibly with an error indication). +.PP +Time zone designations should consist of at least three (3) +and no more than six (6) ASCII characters from the set of +alphanumerics, +.q "\*-", +and +.q "+". +This is for compatibility with POSIX requirements for +time zone abbreviations. +.PP +When reading a version 2 or higher file, readers +should ignore the version 1 header and data block except for +the purpose of skipping over them. +.PP +Readers should calculate the total lengths of the +headers and data blocks and check that they all fit within +the actual file size, as part of a validity check for the file. +.PP +When a positive leap second occurs, readers should append an extra +second to the local minute containing the second just before the leap +second. If this occurs when the UTC offset is not a multiple of 60 +seconds, the leap second occurs earlier than the last second of the +local minute and the minute's remaining local seconds are numbered +through 60 instead of the usual 59; the UTC offset is unaffected. +.SS Common interoperability issues +This section documents common problems in reading or writing TZif files. +Most of these are problems in generating TZif files for use by +older readers. +The goals of this section are: +.IP * 2 +to help TZif writers output files that avoid common +pitfalls in older or buggy TZif readers, +.IP * +to help TZif readers avoid common pitfalls when reading +files generated by future TZif writers, and +.IP * +to help any future specification authors see what sort of +problems arise when the TZif format is changed. +.PP +When new versions of the TZif format have been defined, a +design goal has been that a reader can successfully use a TZif +file even if the file is of a later TZif version than what the +reader was designed for. +When complete compatibility was not achieved, an attempt was +made to limit glitches to rarely used timestamps and allow +simple partial workarounds in writers designed to generate +new-version data useful even for older-version readers. +This section attempts to document these compatibility issues and +workarounds, as well as to document other common bugs in +readers. +.PP +Interoperability problems with TZif include the following: +.IP * 2 +Some readers examine only version 1 data. +As a partial workaround, a writer can output as much version 1 +data as possible. +However, a reader should ignore version 1 data, and should use +version 2+ data even if the reader's native timestamps have only +32 bits. +.IP * +Some readers designed for version 2 might mishandle +timestamps after a version 3 or higher file's last transition, because +they cannot parse extensions to POSIX in the TZ-like string. +As a partial workaround, a writer can output more transitions +than necessary, so that only far-future timestamps are +mishandled by version 2 readers. +.IP * +Some readers designed for version 2 do not support +permanent daylight saving time with transitions after 24:00 +\(en e.g., a TZ string +.q "EST5EDT,0/0,J365/25" +denoting permanent Eastern Daylight Time +(\-04). +As a workaround, a writer can substitute standard time +for two time zones east, e.g., +.q "XXX3EDT4,0/0,J365/23" +for a time zone with a never-used standard time (XXX, \-03) +and negative daylight saving time (EDT, \-04) all year. +Alternatively, +as a partial workaround a writer can substitute standard time +for the next time zone east \(en e.g., +.q "AST4" +for permanent +Atlantic Standard Time (\-04). +.IP * +Some readers designed for version 2 or 3, and that require strict +conformance to RFC 8536, reject version 4 files whose leap second +tables are truncated at the start or that end in expiration times. +.IP * +Some readers ignore the footer, and instead predict future +timestamps from the time type of the last transition. +As a partial workaround, a writer can output more transitions +than necessary. +.IP * +Some readers do not use time type 0 for timestamps before +the first transition, in that they infer a time type using a +heuristic that does not always select time type 0. +As a partial workaround, a writer can output a dummy (no-op) +first transition at an early time. +.IP * +Some readers mishandle timestamps before the first +transition that has a timestamp not less than \-2**31. +Readers that support only 32-bit timestamps are likely to be +more prone to this problem, for example, when they process +64-bit transitions only some of which are representable in 32 +bits. +As a partial workaround, a writer can output a dummy +transition at timestamp \-2**31. +.IP * +Some readers mishandle a transition if its timestamp has +the minimum possible signed 64-bit value. +Timestamps less than \-2**59 are not recommended. +.IP * +Some readers mishandle POSIX-style TZ strings that +contain +.q "<" +or +.q ">". +As a partial workaround, a writer can avoid using +.q "<" +or +.q ">" +for time zone abbreviations containing only alphabetic +characters. +.IP * +Many readers mishandle time zone abbreviations that contain +non-ASCII characters. +These characters are not recommended. +.IP * +Some readers may mishandle time zone abbreviations that +contain fewer than 3 or more than 6 characters, or that +contain ASCII characters other than alphanumerics, +.q "\*-", +and +.q "+". +These abbreviations are not recommended. +.IP * +Some readers mishandle TZif files that specify +daylight-saving time UT offsets that are less than the UT +offsets for the corresponding standard time. +These readers do not support locations like Ireland, which +uses the equivalent of the POSIX TZ string +.q "IST\*-1GMT0,M10.5.0,M3.5.0/1", +observing standard time +(IST, +01) in summer and daylight saving time (GMT, +00) in winter. +As a partial workaround, a writer can output data for the +equivalent of the POSIX TZ string +.q "GMT0IST,M3.5.0/1,M10.5.0", +thus swapping standard and daylight saving time. +Although this workaround misidentifies which part of the year +uses daylight saving time, it records UT offsets and time zone +abbreviations correctly. +.IP * +Some readers generate ambiguous timestamps for positive leap seconds +that occur when the UTC offset is not a multiple of 60 seconds. +For example, in a timezone with UTC offset +01:23:45 and with +a positive leap second 78796801 (1972-06-30 23:59:60 UTC), some readers will +map both 78796800 and 78796801 to 01:23:45 local time the next day +instead of mapping the latter to 01:23:46, and they will map 78796815 to +01:23:59 instead of to 01:23:60. +This has not yet been a practical problem, since no civil authority +has observed such UTC offsets since leap seconds were +introduced in 1972. +.PP +Some interoperability problems are reader bugs that +are listed here mostly as warnings to developers of readers. +.IP * 2 +Some readers do not support negative timestamps. +Developers of distributed applications should keep this +in mind if they need to deal with pre-1970 data. +.IP * +Some readers mishandle timestamps before the first +transition that has a nonnegative timestamp. +Readers that do not support negative timestamps are likely to +be more prone to this problem. +.IP * +Some readers mishandle time zone abbreviations like +.q "\*-08" +that contain +.q "+", +.q "\*-", +or digits. +.IP * +Some readers mishandle UT offsets that are out of the +traditional range of \-12 through +12 hours, and so do not +support locations like Kiritimati that are outside this +range. +.IP * +Some readers mishandle UT offsets in the range [\-3599, \-1] +seconds from UT, because they integer-divide the offset by +3600 to get 0 and then display the hour part as +.q "+00". +.IP * +Some readers mishandle UT offsets that are not a multiple +of one hour, or of 15 minutes, or of 1 minute. +.SH SEE ALSO +.BR time (2), +.BR localtime (3), +.BR tzset (3), +.BR tzselect (8), +.BR zdump (8), +.BR zic (8). +.PP +Olson A, Eggert P, Murchison K. The Time Zone Information Format (TZif). +2019 Feb. +.UR https://\:datatracker.ietf.org/\:doc/\:html/\:rfc8536 +Internet RFC 8536 +.UE +.UR https://\:doi.org/\:10.17487/\:RFC8536 +doi:10.17487/RFC8536 +.UE . |