TIMEGM(3) Linux Programmer's Manual TIMEGM(3)

timegm, timelocal - inverses of gmtime and localtime

#include <time.h>
time_t timelocal(struct tm *tm);
time_t timegm(struct tm *tm);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
timelocal(), timegm():
Since glibc 2.19:
Glibc 2.19 and earlier:

The functions timelocal() and timegm() are the inverses of localtime(3) and gmtime(3). Both functions take a broken-down time and convert it to calendar time (seconds since the Epoch, 1970-01-01 00:00:00 +0000, UTC). The difference between the two functions is that timelocal() takes the local timezone into account when doing the conversion, while timegm() takes the input value to be Coordinated Universal Time (UTC).

On success, these functions return the calendar time (seconds since the Epoch), expressed as a value of type time_t. On error, they return the value (time_t) -1 and set errno to indicate the cause of the error.

The result cannot be represented.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
timelocal (), timegm () Thread safety MT-Safe env locale

These functions are nonstandard GNU extensions that are also present on the BSDs. Avoid their use.

The timelocal() function is equivalent to the POSIX standard function mktime(3). There is no reason to ever use it.

gmtime(3), localtime(3), mktime(3), tzset(3)
2016-12-12 GNU