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

NAME
       tzfile - timezone information

DESCRIPTION
       This page describes the structure of the timezone files used by tzset(3).  These files are
       typically found under one of the directories /usr/lib/zoneinfo or /usr/share/zoneinfo.

       Timezone information files begin with a 44-byte header structured as follows:

       *  The magic four-byte sequence "TZif" identifying this as a timezone information file.

       *  A single character identifying the version of the file's format: either  an  ASCII  NUL
          ('\0') or a '2' (0x32).

       *  Fifteen bytes containing zeros reserved for future use.

       *  Six  four-byte  values of type long, 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 UTC/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 is stored in the file.

          tzh_timecnt
                 The number of "transition times" for which data is stored in the file.

          tzh_typecnt
                 The number of "local time types" for which data is stored in the file (must  not
                 be zero).

          tzh_charcnt
                 The number of characters of "timezone abbreviation strings" stored in the file.

       The  above  header  is  followed  by  tzh_timecnt four-byte values of type long, 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.  Next come tzh_timecnt one-byte values of type 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  ttinfo
       structures  (with  tzh_typecnt entries) that appear next in the file; these structures are
       defined as follows:

           struct ttinfo {
               long         tt_gmtoff;
               int          tt_isdst;
               unsigned int tt_abbrind;
           };

       Each structure is written as a four-byte value for tt_gmtoff of type long, 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  UTC,  tt_isdst
       tells  whether  tm_isdst  should be set by localtime(3), and tt_abbrind serves as an index
       into the array of timezone 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(2)) at which  a  leap  second
       occurs;  the  second  gives the total number of leap seconds to be applied after 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 timezone file is  used  in  handling
       POSIX-style timezone environment variables.

       Finally,  there  are tzh_ttisgmtcnt UTC/local indicators, each stored as a one-byte value;
       they tell whether the transition times associated with local time types were specified  as
       UTC or local time, and are used when a timezone 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.

NOTES
       This manual page documents  in the glibc source archive, see timezone/tzfile.h.

       It seems that timezone  uses  tzfile  internally,  but  glibc  refuses  to  expose  it  to
       userspace.   This  is  most  likely because the standardised functions are more useful and
       portable, and actually documented by glibc.  It may only be in glibc just to  support  the
       non-glibc-maintained timezone data (which is maintained by some other entity).

   Version 2 format
       For  version-2-format  timezone  files,  the above header and data is followed by a second
       header and data, identical in format except that eight bytes are used for each  transition
       time  or  leap-second  time (and that the version byte in the header record is 0x32 rather
       than 0x00).  After the second header and data comes a newline-enclosed,  POSIX-TZ-environ‐
       ment-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).

       The second section of the timezone file consists of another 44-byte header record, identi‐
       cal in structure to the one at the beginning of the file, except that it  applies  to  the
       data  that follows, which is also identical in structure to the first section of the time‐
       zone file, with the following differences:

       *  The transition time values, after the header, are eight-byte values.

       *  In each leap second record, the leap second value is an eight-byte value.  The  accumu‐
          lated leap second count is still a four-byte value.

       In all cases, the eight-byte time values are given in the "standard" byte order, the high-
       order byte first.

   POSIX timezone string
       The second eight-byte time value section is followed by an optional third section: a  sin‐
       gle  ASCII newline character ('\n'), then a text string followed by a second newline char‐
       acter.  The text string is a POSIX timezone string,  whose  format  is  described  in  the
       tzset(3) manual page.

       The  POSIX  timezone  string defines a rule for computing transition times that follow the
       last transition time explicitly specified in the timezone information file.

   Summary of the timezone information file format

              Four-byte value section
              (header version 0x00 or 0x32)
                      Header record
                      Four-byte transition times
                      Transition time index
                      ttinfo structures
                      Timezone abbreviation array
                      Leap second records
                      Standard/Wall array
                      UTC/Local array

              Eight-byte value section
              (only if first header version is 0x32,
              the second header's version is also 0x32)
                      Header record
                      Eight-byte transition times
                      Transition time index
                      ttinfo structures
                      Timezone abbreviation array
                      Leap second records
                      Standard/Wall array
                      UTC/Local array

              Third section
              (optional, only in 0x32 version files)
                      Newline character
                      Timezone string
                      Newline character

SEE ALSO
       ctime(3), tzset(3), tzselect(8),

       timezone/tzfile.h in the glibc source tree

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

                                            2015-05-07                                  TZFILE(5)

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