man3/timegm.3

.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH TIMEGM 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
timegm, timelocal \- inverses of gmtime and localtime
.SH SYNOPSIS
.nf
.B #include <time.h>
.PP
.BI "time_t timelocal(struct tm *" tm );
.BI "time_t timegm(struct tm *" tm );
.PP
.fi
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR timelocal (),
.BR timegm ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
The functions
.BR timelocal ()
and
.BR timegm ()
are the inverses of
.BR localtime (3)
and
.BR 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
.BR timelocal ()
takes the local timezone into account when doing the conversion, while
.BR timegm ()
takes the input value to be Coordinated Universal Time (UTC).
.SH RETURN VALUE
On success,
these functions return the calendar time (seconds since the Epoch),
expressed as a value of type
.IR time_t .
On error, they return the value
.I (time_t)\ \-1
and set
.I errno
to indicate the error.
.SH ERRORS
.TP
.B EOVERFLOW
The result cannot be represented.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR timelocal (),
.BR timegm ()
T}	Thread safety	MT-Safe env locale
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
These functions are nonstandard GNU extensions
that are also present on the BSDs.
Avoid their use.
.SH NOTES
The
.BR timelocal ()
function is equivalent to the POSIX standard function
.BR mktime (3).
There is no reason to ever use it.
.SH SEE ALSO
.BR gmtime (3),
.BR localtime (3),
.BR mktime (3),
.BR tzset (3)