MKTIME(P) POSIX Programmer's Manual MKTIME(P)
mktime - convert broken-down time into time since the Epoch
time_t mktime(struct tm *timeptr);
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 <time.h>.
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 tzset().
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
IEEE Std 1003.1-2001, Section 4.14, Seconds Since the Epoch) corrected for timezone and
any seasonal time adjustments, where the names in the structure and in the expression cor-
Upon successful completion, the values of the tm_wday and tm_yday components of the struc-
ture shall be set appropriately, and the other components are set to represent the speci-
fied 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
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.
No errors are defined.
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);
asctime() , clock() , ctime() , difftime() , gmtime() , localtime() , strftime() , strp-
time() , time() , utime() , the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 MKTIME(P)