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  the  magic  characters  "TZif"  to  identify  them  as  timezone
       information  files,  followed  by  a  character identifying the version of the file's format (as of 2005,
       either an ASCII NUL ('\0') or a '2') followed by fifteen bytes containing zeros reserved for future  use,
       followed by 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 timezone 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 <tzfile.h> 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).

SEE ALSO

       time(3), gettimeofday(3), tzset(3), ctime(3)

       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.  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).

SEE ALSO

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

       timezone/tzfile.h in the glibc source tree

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

                                                   2012-05-04                                          TZFILE(5)