PROLOG

       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.

NAME

       daylight, timezone, tzname, tzset — set timezone conversion information

SYNOPSIS

       #include <time.h>

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

DESCRIPTION

       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.

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       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    │
                                       └───────────┴────────────┘

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.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

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

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

COPYRIGHT

       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 .