tzset - Man Page

set timezone conversion information


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <time.h>

extern int daylight;
extern long timezone;
extern char *tzname[2];
void tzset(void);


The tzset() function shall use the value of the environment variable TZ to set time conversion information used by ctime(), localtime(), mktime(), and strftime(). If TZ is absent from the environment, implementation-defined default timezone information shall be used.

The tzset() function shall set the external variable tzname as follows:

tzname[0] = "std";
tzname[1] = "dst";

where std and dst are as described in the Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables.

The tzset() function also shall set the external variable daylight to 0 if Daylight Savings Time conversions should never be applied for the timezone in use; otherwise, non-zero. The external variable timezone shall be set to the difference, in seconds, between Coordinated Universal Time (UTC) and local standard time.

If a thread accesses tzname, daylight, or timezone directly while another thread is in a call to tzset(), or to any function that is required or allowed to set timezone information as if by calling tzset(), the behavior is undefined.

Return Value

The tzset() function shall not return a value.


No errors are defined.

The following sections are informative.


Example TZ variables and their timezone differences are given in the table below:


Application Usage

Since the ctime(), localtime(), mktime(), strftime(), and strftime_l() functions are required to set timezone information as if by calling tzset(), there is no need for an explicit tzset() call before using these functions. However, portable applications should call tzset() explicitly before using ctime_r() or localtime_r() because setting timezone information is optional for those functions.



Future Directions


See Also

ctime(), localtime(), mktime(), strftime()

The Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables, <time.h>

Referenced By

localtime(3p), mktime(3p), strftime(3p), time.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual