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TZSET(3)                            Linux Programmer's Manual                            TZSET(3)

       tzset, tzname, timezone, daylight - initialize time conversion information

       #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  sav‐
       ing time rules, or to nonzero if there is a time during the year when daylight saving time

       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 can‐
       not 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  fol‐
       lowing 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 60.

       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 02:00:00.

       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


       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  speci‐
       fies  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:


       Under  glibc,  the system timezone directory is determined using the TZDIR the environment
       variable.   If  TZDIR  the  default  depends  on  the  system  setup,  but   is   normally

       This timezone directory contains the files

           localtime      local timezone file
           posixrules     rules for POSIX-style TZ's

       Often,  /etc/localtime is a symbolic link 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 get‐
       timeofday(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)

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

                                            2014-08-19                                   TZSET(3)

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