.TH TZFILE 5 .SH NAME tzfile \- time zone information .SH SYNOPSIS .B #include .SH DESCRIPTION The time zone information files used by .IR tzset (3) begin with the magic characters "TZif" to identify them as time zone information files, followed by a character identifying the version of the file's format (as of 2013, either an ASCII NUL, or '2', or '3') followed by fifteen bytes containing zeroes reserved for future use, followed by six four-byte values of type .BR long , written in a ``standard'' byte order (the high-order byte of the value is written first). These values are, in order: .TP .I tzh_ttisgmtcnt The number of UT/local indicators stored in the file. .TP .I tzh_ttisstdcnt The number of standard/wall indicators stored in the file. .TP .I tzh_leapcnt The number of leap seconds for which data is stored in the file. .TP .I tzh_timecnt The number of "transition times" for which data is stored in the file. .TP .I tzh_typecnt The number of "local time types" for which data is stored in the file (must not be zero). .TP .I tzh_charcnt The number of characters of "time zone abbreviation strings" stored in the file. .PP The above header is followed by .I tzh_timecnt four-byte values of type .BR long , sorted in ascending order. These values are written in ``standard'' byte order. Each is used as a transition time (as returned by .IR time (2)) at which the rules for computing local time change. Next come .I tzh_timecnt one-byte values of type .BR "unsigned char" ; each one tells which of the different types of ``local time'' types described in the file is associated with the same-indexed transition time. These values serve as indices into an array of .I ttinfo structures (with .I tzh_typecnt entries) that appears next in the file; these structures are defined as follows: .in +.5i .sp .nf .ta .5i +\w'unsigned int\0\0'u struct ttinfo { long tt_gmtoff; int tt_isdst; unsigned int tt_abbrind; }; .in -.5i .fi .sp Each structure is written as a four-byte value for .I tt_gmtoff of type .BR long , in a standard byte order, followed by a one-byte value for .I tt_isdst and a one-byte value for .IR tt_abbrind . In each structure, .I tt_gmtoff gives the number of seconds to be added to UT, .I tt_isdst tells whether .I tm_isdst should be set by .I localtime (3) and .I tt_abbrind serves as an index into the array of time zone abbreviation characters that follow the .I ttinfo structure(s) in the file. .PP Then there are .I tzh_leapcnt pairs of four-byte values, written in standard byte order; the first value of each pair gives the time (as returned by .IR time(2)) at which a leap second occurs; the second gives the .I total number of leap seconds to be applied after the given time. The pairs of values are sorted in ascending order by time. .PP Then there are .I tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as standard time or wall clock time, and are used when a time zone file is used in handling POSIX-style time zone environment variables. .PP Finally there are .I tzh_ttisgmtcnt UT/local indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as UT or local time, and are used when a time zone file is used in handling POSIX-style time zone environment variables. .PP .I Localtime uses the first standard-time .I ttinfo structure in the file (or simply the first .I ttinfo structure in the absence of a standard-time structure) if either .I tzh_timecnt is zero or the time argument is less than the first transition time recorded in the file. .PP For version-2-format time zone 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. 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 (with nothing between the newlines if there is no POSIX representation for such instants). .PP For version-3-format time zone files, the POSIX-TZ-style string may use two minor extensions to the POSIX TZ format, as described in .IR newtzset (3). First, the hours part of its transition times may be signed and range from \(mi167 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. .PP Also, for version-3-format time zone files, the version-2 header and data are optionally followed by a section containing auxiliary meta-information that is not needed to process time stamps. This section, if present, consists of the four magic bytes "=TZ\en", followed by zero or more newline-terminated byte strings, followed by another copy of the newline-enclosed POSIX-TZ-style string (this last is for the benefit of any older clients that look for the TZ string at the very end of the file). Each newline-terminated byte string consists of a name-value pair separated by "=" and terminated by newline. Names consist of ASCII letters, digits and underscores, and start with a letter; duplicate names are not allowed. Two common names are "name", the Zone name for the data, and "version", the version number. Values consist of any bytes except NUL, newline, and backslash; however, newline and backslash can represented via the two-byte strings "\en" and "\e\e" respectively. .SH SEE ALSO newctime(3), newtzset(3) .\" This file is in the public domain, so clarified as of .\" 1996-06-05 by Arthur David Olson.