This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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(3POSIX)

       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, Chapter8, EnvironmentVariables.

       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.

       No errors are defined.

       Thefollowingsectionsareinformative.

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

       None.

       daylight, timezone, tzname, tzset — 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.

       None.

       The tzset() function shall not return a value.

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

       The Base Definitions volume of POSIX.1‐2017, Chapter8, EnvironmentVariables, <time.h>

       #include <time.h>

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