NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       tzname:
           _POSIX_C_SOURCE

       timezone, daylight:
           _XOPEN_SOURCE
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE

DESCRIPTION

       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).

       The tzset() function initializes these variables to unspecified values if this timezone is
       a geographical timezone like "America/New_York" (see below).

       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.

       A nonempty value of TZ can be one of two formats, either of which can  be  preceded  by  a
       colon  which  is  ignored.   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 character 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:

           [+|-]hh[:mm[:ss]]

       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 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.  They use the same format as offset except that the hour can be in the
       range [-167, 167] to represent times before and after the  named  day.   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  September's  last
       Sunday, at the default time 02:00:00, to April's first Sunday at 03:00:00.

           TZ="NZST-12:00:00NZDT-13:00:00,M9.5.0,M4.1.0/3"

       The  second  —or  "geographical"— format specifies that the timezone information should be
       read from a file:

           filespec

       The filespec specifies a 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 specified file cannot  be  read  or  interpreted,  Coordinated
       Universal  Time  (UTC) is used; however, applications should not depend on random filespec
       values standing for UTC, as TZ formats may be extended in the future.

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

           TZ="Pacific/Auckland"

ENVIRONMENT

       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.

FILES

       /etc/localtime
              The system timezone file.

       /usr/share/zoneinfo/
              The system timezone database directory.

       /usr/share/zoneinfo/posixrules
              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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌────────────────────────────────────────────────────┬───────────────┬────────────────────┐
       │Interface                                           │ Attribute     │ Value              │
       ├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
       │tzset()                                             │ Thread safety │ MT-Safe env locale │
       └────────────────────────────────────────────────────┴───────────────┴────────────────────┘

STANDARDS

       POSIX.1-2024.

HISTORY

       tzset()
       tzname POSIX.1-1988, SVr4, 4.3BSD.

       timezone
       daylight
              POSIX.1-2001 (XSI), 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.

CAVEATS

       Because  the values of tzname, timezone, and daylight are often unspecified, and accessing
       them can lead to undefined behavior in multithreaded  applications,  code  should  instead
       obtain  time  zone  offset and abbreviations from the tm_gmtoff and tm_zone members of the
       broken-down time structure tm(3type).

SEE ALSO

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