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.
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements described
here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 defers to the ISO C standard.
The mktime() function shall convert the broken-down time, expressed
as local time, in the structure pointed to by timeptr, into a time
since the Epoch value with the same encoding as that of the values
returned by time(). The original values of the tm_wday and tm_yday
components of the structure are ignored, and the original values of
the other components are not restricted to the ranges described in
A positive or 0 value for tm_isdst shall cause mktime() to presume
initially that Daylight Savings Time, respectively, is or is not in
effect for the specified time. A negative value for tm_isdst shall
cause mktime() to attempt to determine whether Daylight Savings Time
is in effect for the specified time.
Local timezone information shall be set as though mktime() called
The relationship between the tm structure (defined in the <time.h>
header) and the time in seconds since the Epoch is that the result
shall be as specified in the expression given in the definition of
seconds since the Epoch (see the Base Definitions volume of
POSIX.1‐2008, Section 4.15, Seconds Since the Epoch) corrected for
timezone and any seasonal time adjustments, where the names in the
structure and in the expression correspond.
Upon successful completion, the values of the tm_wday and tm_yday
components of the structure shall be set appropriately, and the other
components are set to represent the specified time since the Epoch,
but with their values forced to the ranges indicated in the <time.h>
entry; the final value of tm_mday shall not be set until tm_mon and
tm_year are determined.
The mktime() function shall return the specified time since the Epoch
encoded as a value of type time_t. If the time since the Epoch
cannot be represented, the function shall return the value (time_t)−1
and set errno to indicate the error.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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
IEEE/The Open Group 2013 MKTIME(3P)