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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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TZSET(3P) POSIX Programmer's Manual TZSET(3P)
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.
daylight, timezone, tzname, tzset — set timezone conversion
information
#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(3p),
localtime(3p), mktime(3p), and strftime(3p). 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.
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:
┌────────────┬────────────┐
│ TZ │ timezone │
├────────────┼────────────┤
│ EST5EDT │ 5*60*60 │
│ GMT0 │ 0*60*60 │
│ JST-9 │ -9*60*60 │
│ MET-1MEST │ -1*60*60 │
│ MST7MDT │ 7*60*60 │
│ PST8PDT │ 8*60*60 │
└────────────┴────────────┘
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.
None.
None.
ctime(3p), localtime(3p), mktime(3p), strftime(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, time.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 TZSET(3P)
Pages that refer to this page: time.h(0p), daylight(3p), localtime(3p), mktime(3p), strftime(3p), timezone(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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