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       tzset,   tzname,   timezone,  daylight  -  initialize  time  conversion


       #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 >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE


       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  timezone.   In  a
       System-V-like  environment,  it  will  also  set the variables timezone
       (seconds West of UTC) and daylight (to 0 if this timezone does not have
       any daylight saving time rules, or to nonzero if there is a time during
       the year when daylight saving time applies).

       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
       empty 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 timezone:

              std offset

       The std string specifies the name of the timezone 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
       timezone 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 timezone, as described above.  The dst string  and
       offset  specify  the  name  and  offset  for the corresponding daylight
       saving timezone.  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.
              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

       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 timezone information should be read
       from a file:


       If the file specification filespec is omitted, the timezone 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 timezone information from.  If filespec does not begin
       with  a  '/', the file specification is relative to the system timezone

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



       The system timezone 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 timezone file
       posixrules     rules for POSIX-style TZ's

       Often /etc/localtime is a symlink to  the  file  localtime  or  to  the
       correct timezone file in the system timezone 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  timezone  corresponding to its first argument (minutes West of
       UTC).  If the second argument  was  0,  the  standard  name  was  used,
       otherwise the daylight saving time version.


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


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       be found at

                                  2010-02-25                          TZSET(3)