Sets time environment variables.


This API cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported in Universal Windows Platform apps.


void _tzset( void );


The _tzset function uses the current setting of the environment variable TZ to assign values to three global variables: _daylight, _timezone, and _tzname. These variables are used by the _ftime and localtime functions to make corrections from coordinated universal time (UTC) to local time, and by the time function to compute UTC from system time. Use the following syntax to set the TZ environment variable:

set TZ=tzn [+|-]hh[:mm[:ss] ][dzn]

Three-letter time-zone name, such as PST. You must specify the correct offset from local time to UTC.

Difference in hours between UTC and local time. Sign (+) optional for positive values.

Minutes. Separated from hh by a colon (:).

Seconds. Separated from mm by a colon (:).

Three-letter daylight-saving-time zone such as PDT. If daylight saving time is never in effect in the locality, set TZ without a value for dzn. The C run-time library assumes the United States' rules for implementing the calculation of daylight saving time (DST).


Take care in computing the sign of the time difference. Because the time difference is the offset from local time to UTC (rather than the reverse), its sign may be the opposite of what you might intuitively expect. For time zones ahead of UTC, the time difference is negative; for those behind UTC, the difference is positive.

For example, to set the TZ environment variable to correspond to the current time zone in Germany, enter this command on the command line:


This command uses GST to indicate German standard time. It assumes that UTC is one hour behind Germany (or in other words, that Germany is one hour ahead of UTC). And, it assumes that Germany observes daylight-saving time.

If the TZ value isn't set, _tzset attempts to use the time zone information specified by the operating system. In the Windows operating system, this information is specified in the Date/Time application in Control Panel. If _tzset can't obtain this information, it uses PST8PDT by default, which signifies the Pacific Time zone.

Based on the TZ environment variable value, the following values are assigned to the global variables _daylight, _timezone, and _tzname when _tzset is called:

Global variable Description Default value
_daylight Nonzero value if a daylight-saving-time zone is specified in TZ setting; otherwise, 0. 1
_timezone Difference in seconds between local time and UTC. 28800 (28,800 seconds equals 8 hours)
_tzname[0] String value of time-zone name from TZ environmental variable; empty if TZ hasn't been set. PST
_tzname[1] String value of daylight-saving-time zone; empty if daylight-saving-time zone is omitted from TZ environmental variable. PDT

The default values shown in the preceding table for _daylight and the _tzname array correspond to "PST8PDT." If the DST zone is omitted from the TZ environmental variable, the value of _daylight is 0 and the _ftime, gmtime, and localtime functions return 0 for their DST flags.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.


Routine Required header
_tzset <time.h>

The _tzset function is Microsoft-specific. For more information, see Compatibility.


// crt_tzset.cpp
// This program uses _tzset to set the global variables
// named _daylight, _timezone, and _tzname. Since TZ is
// not being explicitly set, it uses the system time.

#include <time.h>
#include <stdlib.h>
#include <stdio.h>

int main( void )
    int daylight;
    _get_daylight( &daylight );
    printf( "_daylight = %d\n", daylight );
    long timezone;
    _get_timezone( &timezone );
    printf( "_timezone = %ld\n", timezone );
    size_t s;
    char tzname[100];
    _get_tzname( &s, tzname, sizeof(tzname), 0 );
    printf( "_tzname[0] = %s\n", tzname );
    exit( 0 );
_daylight = 1
_timezone = 28800
_tzname[0] = Pacific Standard Time

See also

Time management
asctime, _wasctime
_ftime, _ftime32, _ftime64
gmtime, _gmtime32, _gmtime64
localtime, _localtime32, _localtime64
time, _time32, _time64
_utime, _utime32, _utime64, _wutime, _wutime32, _wutime64