NAME

       tzset - initialize time conversion information

SYNOPSIS

       #include <time.h>

       void tzset (void);

       extern char *tzname[2]

DESCRIPTION

       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:

              :[filespec]

       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 directory.

FILES

       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.

CONFORMING TO

       SVID 3, POSIX, BSD 4.3

SEE ALSO

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