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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)