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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 || _XOPEN_SOURCE
       tzname: _POSIX_C_SOURCE || _XOPEN_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  time  zone.   In  a
       SysV-like  environment it will also set the variables timezone (seconds
       West of GMT) and daylight (0 if  this  time  zone  does  not  have  any
       daylight  saving time rules, 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 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

       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
       saving  time  zone.   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 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 directory.

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



       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.


       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 time zone corresponding to its first argument (minutes  West  of
       GMT).   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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                                  2007-07-26                          TZSET(3)