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TZSET(3)                   Linux Programmer's Manual                  TZSET(3)

       tzset, tzname, timezone, daylight - initialize time conversion informa-

       #include <time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       tzset(): _POSIX_C_SOURCE || _XOPEN_SOURCE
       tzname: _POSIX_C_SOURCE || _XOPEN_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE

       The tzset() function initializes the tzname variable from the TZ  envi-
       ronment  variable.   This function is automatically called by the other
       time conversion functions that depend on the time zone.  In a SysV-like
       environment  it  will  also set the variables timezone (seconds West of
       GMT) and daylight (0 if this time zone does not have any daylight  sav-
       ing  time  rules, non-zero if there is a time during the year when day-
       light saving time applies).

       If the TZ variable does not appear in the environment, the tzname vari-
       able  is  initialized  with  the best approximation of local wall clock
       time, as specified by the tzfile(5)-format file localtime found in  the
       system   timezone   directory   (see  below).   (One  also  often  sees
       /etc/localtime used here, a symlink to the right  file  in  the  system
       timezone directory.)

       If  the  TZ  variable  does  appear in the environment but its value is
       empty or its value cannot be interpreted using any of the formats spec-
       ified below, Coordinated Universal Time (UTC) is used.

       The  value of TZ can be one of three formats.  The first format is used
       when there is no daylight saving time in the local time zone:

              std offset

       The std string specifies the name of the time zone and must be three or
       more  alphabetic characters.  The offset string immediately follows std
       and specifies the time value to be added to the local time to get Coor-
       dinated Universal Time (UTC).  The offset is positive if the local time
       zone is west of the Prime Meridian and negative if  it  is  east.   The
       hour must be between 0 and 24, and the minutes and seconds 0 and 59.

       The second format is used when there is daylight saving time:

              std offset dst [offset],start[/time],end[/time]

       There  are  no spaces in the specification.  The initial std and offset
       specify the standard time zone, as described above.  The dst string and
       offset  specify the name and offset for the corresponding daylight sav-
       ing time zone.  If the offset is omitted, it default to one hour  ahead
       of standard time.

       The  start  field  specifies when daylight saving time goes into effect
       and the end field specifies when the change is made  back  to  standard
       time.  These fields may have the following formats:

       Jn     This  specifies the Julian day with n between 1 and 365.  Febru-
              ary 29 is never counted even in leap years.

       n      This specifies the Julian day with n between 1 and 365.   Febru-
              ary 29 is counted in leap years.

       Mm.w.d This  specifies  day  d (0 <= d <= 6) of week w (1 <= w <= 5) of
              month m (1 <= m <= 12).  Week 1 is the first week in which day d
              occurs and week 5 is the last week in which day d occurs.  Day 0
              is a Sunday.

       The time fields specify when, in the local time  currently  in  effect,
       the  change  to  the  other  time  occurs.   If omitted, the default is

       Here is an example for New Zealand, where the standard time  (NZST)  is
       12  hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead
       of UTC, runs from the first Sunday in October to the  third  Sunday  in
       March, and the changeovers happen at the default time of 02:00:00:


       The  third  format  specifies  that the time zone information should be
       read from a file:


       If the file specification filespec is omitted, the time  zone  informa-
       tion  is read from the file localtime in the system timezone directory,
       which  nowadays  usually  is  /usr/share/zoneinfo.   This  file  is  in
       tzfile(5)   format.    If  filespec  is  given,  it  specifies  another
       tzfile(5)-format file to read the time zone information from.  If file-
       spec  does  not begin with a '/', the file specification is relative to
       the system timezone directory.

       Here's an example, once more for New Zealand:


       The system time zone directory used depends  on  the  (g)libc  version.
       Libc4 and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this
       doesn't work, will try /usr/share/zoneinfo.  Glibc2 will use the  envi-
       ronment  variable  TZDIR, when that exists.  Its default depends on how
       it was installed, but normally is /usr/share/zoneinfo.

       This timezone directory contains the files
       localtime      local time zone file
       posixrules     rules for POSIX-style TZ's

       Often /etc/localtime is a symlink to the file localtime or to the  cor-
       rect time zone file in the system time zone directory.

       SVr4, POSIX.1-2001, 4.3BSD.

       Note  that the variable daylight does not indicate that daylight saving
       time applies right now.  It used to give the number of  some  algorithm
       (see the variable tz_dsttime in gettimeofday(2)).  It has been obsolete
       for many years but is required by SUSv2.

       4.3BSD had a function char *timezone(zone, dst) that returned the  name
       of  the  time zone corresponding to its first argument (minutes West of
       GMT).  If the second argument was 0, the standard name was used, other-
       wise the daylight saving time version.

       date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

                                  2007-07-26                          TZSET(3)

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