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

       tzset - initialize time conversion information

       #include <time.h>

       void tzset (void);

       extern char *tzname[2]

       The tzset() function initializes the tzname variable from the TZ
       environment variable.  This function is automatically called by the other
       time conversion functions that depend on the time zone.

       If the TZ variable does not appear in the environment, the tzname
       variable 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 NULL
       or its value cannot be interpreted using any of the formats specified
       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
       Coordinated 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 savings
       time zone.  If the offset is omitted, it defaults  to one hour ahead of
       standard time.

       The start field specifies when daylight savings 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.  February
              29 is never counted even in leap years.

       n      This specifies the Julian day with n between 1 and 365.  February
              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 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 information
       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 filespec does not begin with
       a `/', the file specification is relative to the system timezone

       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
       environment 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 correct
       time zone file in the system time zone directory.

       SVID 3, POSIX, BSD 4.3

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

BSD                               July 2, 1993                          TZSET(3)