tzset(3)

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
 | 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)