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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, past,
       present or future when daylight saving time applies).

       If the TZ variable does not  appear  in  the  environment,  the  system
       timezone  is  used.   The  system timezone is configured by copying, or
       linking, a file in the tzfile(5) format to /etc/localtime.  A  timezone
       database of these files may be located in the system timezone directory
       (see the FILES section below).

       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, then Coordinated Universal Time (UTC) is used.

       The value of TZ can be one of two  formats.   The  first  format  is  a
       string of characters that directly represent the timezone to be used:

              std offset[dst[offset][,start[/time],end[/time]]]

       There  are no spaces in the specification.  The std string specifies an
       abbreviation for the timezone and must  be  three  or  more  alphabetic
       characters.   When  enclosed between the less-than (<) and greater-than
       (>) signs, the characters set is expanded to include the plus (+) sign,
       the  minus (-) sign, and digits.  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 00 and 59:


       The  dst  string  and  offset  specify  the  name  and  offset  for the
       corresponding daylight saving timezone.  If the offset is  omitted,  it
       defaults 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.  Leap
              days are not counted.  In this  format,  February  29  can't  be
              represented;  February  28  is day 59, and March 1 is always day

       n      This specifies the zero-based Julian day with n  between  0  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  second  format  specifies  that the timezone information should be
       read from a file:


       If the file specification filespec is omitted, or its value  cannot  be
       interpreted,  then  Coordinated  Universal  Time  (UTC)  is  used.   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  directory.
       If the colon is omitted each of the above TZ formats will be tried.

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



       TZ     If  this  variable  is  set  its value takes precedence over the
              system configured timezone.

       TZDIR  If this variable is set its  value  takes  precedence  over  the
              system configured timezone database directory path.


              The system timezone file.

              The system timezone database directory.

              When  a  TZ  string  includes  a  dst  timezone without anything
              following it, then this file is used for  the  start/end  rules.
              It  is  in  the  tzfile(5)  format.   By  default,  the zoneinfo
              Makefile hard links it to the America/New_York tzfile.

       Above  are  the  current  standard  file  locations,   but   they   are
       configurable when glibc is compiled.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue              │
       │tzset()   │ Thread safety │ MT-Safe env locale │


       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.


       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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                                  2015-12-28                          TZSET(3)