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  automati­
       cally  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 time­
       zone  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 min­
       utes 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


       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 Sun­
              day.

       The time fields specify when, in the local time  currently
       in  effect, the change to the other time occurs.  If omit­
       ted, 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)-for­
       mat 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.


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.

       SVID 3, POSIX, BSD 4.3


SEE ALSO

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


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