tzfile(5) - NetBSD Manual Pages

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TZFILE(5)                 NetBSD File Formats Manual                 TZFILE(5)


NAME

     tzfile -- time zone information


DESCRIPTION

     The time zone information files used by tzset(3) begin with the magic
     characters ``TZif'' to identify them as time zone information files, fol-
     lowed by a character identifying the version of the file's format (as of
     2013, either an ASCII NUL or a '2', or '3') followed by fifteen bytes
     containing zeroes reserved for future use, followed by six four-byte val-
     ues of type long, followed by six four-byte integer values written in a
     standard byte order (the high-order byte of the value is written first).
     These values are, in order:
     tzh_ttisgmtcnt
             The number of UT/local indicators stored in the file.
     tzh_ttisstdcnt
             The number of standard/wall indicators stored in the file.
     tzh_leapcnt
             The number of leap seconds for which data entries are stored in
             the file.
     tzh_timecnt
             The number of transition times for which data entries are stored
             in the file.
     tzh_typecnt
             The number of local time types for which data entries are stored
             in the file (must not be zero).
     tzh_charcnt
             The number of characters of time zone abbreviation strings stored
             in the file.

     The above header is followed by tzh_timecnt four-byte signed integer val-
     ues sorted in ascending order.  These values are written in These values
     are written in standard byte order.  Each is used as a transition time
     (as returned by time(3)) at which the rules for computing local time
     change.  Next come tzh_timecnt one-byte unsigned integer values; each one
     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.  These values serve as indices into an array of ttinfo
     structures (with tzh_typecnt entries) that appears next in the file;
     these structures are defined as follows:

     struct ttinfo {
             int32_t         tt_gmtoff;
             unsigned char   tt_isdst;
             unsigned char   tt_abbrind;
     };
     Each structure is written as a four-byte signed integer value for
     tt_gmtoff in a standard byte order, followed by a one-byte value for
     tt_isdst and a one-byte value for tt_abbrind.  In each structure,
     tt_gmtoff gives the number of seconds to be added to UT, tt_isdst tells
     whether tm_isdst should be set by localtime(3) and tt_abbrind serves as
     an index into the array of time zone abbreviation characters that follow
     the ttinfo structure(s) in the file.

     Then there are tzh_leapcnt pairs of four-byte values, written in standard
     byte order; the first value of each pair gives the time (as returned by
     time(3)) at which a leap second occurs; the second gives the total number
     of leap seconds to be applied during the time period starting at the
     given time.  The pairs of values are sorted in ascending order by time.

     Then there are 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.

     Finally there are 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.

     localtime(3) uses the first standard-time ttinfo structure in the file
     (or simply the first ttinfo structure in the absence of a standard-time
     structure) if either tzh_timecnt is zero or the time argument is less
     than the first transition time recorded in the file.

     For version-2-format time zone files, the above header and data are fol-
     lowed 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 transi-
     tion time stored in the file (with nothing between the newlines if there
     is no POSIX representation for such instants).  The POSIX-style string
     must must agree with the local time type after both data's last transi-
     tion times; for example, given the string ``WET0WEST,M3.5.0,M10.5.0/3''
     then if a last transition time is in July, the transition's local time
     type must specify a daylight-saving time abbreviated ``WEST'' that is one
     hour east of UT.

     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 tzset(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 day-
     light saving and standard time.

     Future changes to the format may append more data.


SEE ALSO

     ctime(3), localtime(3), time(3), tzset(3), zdump(8) zic(8)

NetBSD 8.0                      October 6, 2016                     NetBSD 8.0