.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
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.TH TIMEGM 3 2001-12-26 "GNU" "Linux Programmer's Manual"
.SH NAME
timegm, timelocal \- inverses for gmtime and localtime
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.BI "time_t timelocal (struct tm *" tm );
.sp
.BI "time_t timegm (struct tm *" tm );
.SH DESCRIPTION
The functions
.B timelocal()
and
.B timegm()
are the inverses to
.BR localtime (3)
and
.BR gmtime (3).
.SH NOTES
These functions are GNU extensions.
The
.B timelocal()
function is equivalent to the POSIX standard function
.BR mktime (3).
There is no reason to ever use it.
.LP
For a portable version of
.BR timegm() ,
set the
.B TZ
environment variable to UTC, call
.B mktime()
and restore the value of
.BR TZ .
Something like

.RS
.nf
#include <time.h>
#include <stdlib.h>

time_t my_timegm (struct tm *tm) {
    time_t ret;
    char *tz;

    tz = getenv("TZ");
    setenv("TZ", "", 1);
    tzset();
    ret = mktime(tm);
    if (tz)
        setenv("TZ", tz, 1);
    else
        unsetenv("TZ");
    tzset();
    return ret;
}
.fi
.RE
.SH "SEE ALSO"
.BR gmtime (3),
.BR localtime (3),
.BR mktime (3),
.BR tzset (3)