NAME | DESCRIPTION | SEE ALSO | COLOPHON

TZFILE(5)                 Linux Programmer's Manual                TZFILE(5)

NAME         top

       tzfile - timezone information

DESCRIPTION         top

       The timezone information files used by tzset(3) are typically found
       under a directory with a name like /usr/share/zoneinfo.  These files
       begin with a 44-byte header containing the following fields:

       * The magic four-byte ASCII sequence “TZif” identifies the file as a
         timezone information file.

       * A byte identifying the version of the file's format (as of 2017,
         either an ASCII NUL, or “2”, or “3”).

       * Fifteen bytes containing zeros reserved for future use.

       * 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 bytes of time zone abbreviation strings stored
                in the file.

       The above header is followed by the following fields, whose lengths
       depend on the contents of the header:

       * tzh_timecnt four-byte signed integer values sorted in ascending
         order.  These values are written in standard byte order.  Each is
         used as a transition time (as returned by time(2)) at which the
         rules for computing local time change.

       * 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.

       * tzh_typecnt ttinfo entries, each 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 bytes that follow the ttinfo structure(s) in the file.

       * tzh_leapcnt pairs of four-byte values, written in standard byte
         order; the first value of each pair gives the nonnegative time (as
         returned by time(2)) 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.  Each transition is for one leap
         second, either positive or negative; transitions always separated
         by at least 28 days minus 1 second.

       * 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 timezone file is used in handling POSIX-style
         timezone environment variables.

       * 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
         timezone file is used in handling POSIX-style timezone environment
         variables.

       The localtime(3) function 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.

   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 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
       “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.  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.

   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
       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.

       Future changes to the format may append more data.

SEE ALSO         top

       time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8)

COLOPHON         top

       This page is part of release 5.00 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

                                 2019-03-06                        TZFILE(5)

Pages that refer to this page: tzset(3)localtime(5)tzselect(8)zdump(8)zic(8)