NAME

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

SYNOPSIS

#include <time.h>



[XSI] extern int daylight;
extern long timezone;


[CX] 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 XBD 8. Environment Variables.

^[XSI] The tzset() function also shall set the external variable daylight to 0 if Daylight Saving 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, ^[XSI] 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.

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 localtime_r() because setting timezone information is optional for that function.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

ctime, localtime, mktime, strftime

XBD 8. Environment Variables, <time.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The example is corrected.

Issue 7

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0377 [880] is applied.

Issue 8

Austin Group Defect 1253 is applied, changing "Daylight Savings" to "Daylight Saving".

Austin Group Defect 1410 is applied, removing the ctime_r() function.