plucky (3) timezone.3.gz

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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).

       ┌───────────────────────────────────────────────────────────────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├───────────────────────────────────────────────────────────────────┼───────────────┼────────────────────┤
       │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)