MKTIME(3P) POSIX Programmer's Manual MKTIME(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
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NAMEmktime — convert broken-down time into time since the Epoch
time_t mktime(struct tm *timeptr);
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 com‐
ponents are not restricted to the ranges described in <time.h>.
A positive or 0 value for tm_isdst shall cause mktime() to presume ini‐
tially 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 sec‐
onds 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
Upon successful completion, the values of the tm_wday and tm_yday com‐
ponents of the structure shall be set appropriately, and the other com‐
ponents 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
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.
The mktime() function shall fail if:
The result cannot be represented.
The following sections are informative.
What day of the week is July 4, 2001?
struct tm time_str;
time_str.tm_year = 2001 — 1900;
time_str.tm_mon = 7 — 1;
time_str.tm_mday = 4;
time_str.tm_hour = 0;
time_str.tm_min = 0;
time_str.tm_sec = 1;
time_str.tm_isdst = −1;
if (mktime(&time_str) == -1)
(void)strftime(daybuf, sizeof(daybuf), "%A", &time_str);
SEE ALSOasctime(), clock(), ctime(), difftime(), gmtime(), localtime(), strf‐
time(), strptime(), time(), tzset(), utime()
The Base Definitions volume of POSIX.1‐2008, Section 4.15, Seconds
Since the Epoch, <time.h>
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 Electri‐
cal 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 .
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IEEE/The Open Group 2013 MKTIME(3P)