The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


mktime - convert broken-down time into time since the Epoch


#include <time.h>

time_t mktime(struct tm *


[CX] [Option Start] 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.1-2017 defers to the ISO C standard. [Option End]

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 shall be ignored, and the original values of the other components shall not be restricted to the ranges described in <time.h>.

[CX] [Option Start] 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 XBD Seconds Since the Epoch) corrected for timezone and any seasonal time adjustments, where the names other than tm_yday in the structure and in the expression correspond, and the tm_yday value used in the expression is the day of the year from 0 to 365 inclusive, calculated from the other tm structure members specified in <time.h> (excluding tm_wday). [Option End]

Upon successful completion, the values of the tm_wday and tm_yday components of the structure shall be set appropriately, and the other components shall be 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 [CX] [Option Start]  and set errno to indicate the error. [Option End]


The mktime() function shall fail if:

[CX] [Option Start] The result cannot be represented. [Option End]

The following sections are informative.


What day of the week is July 4, 2001?

#include <stdio.h>
#include <time.h>

struct tm time_str;
char daybuf[20];
int main(void) { 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)puts("-unknown-"); else { (void)strftime(daybuf, sizeof(daybuf), "%A", &time_str); (void)puts(daybuf); } return 0; }








asctime, clock, ctime, difftime, gmtime, localtime, strftime, strptime, time, tzset, utime

XBD Seconds Since the Epoch, <time.h>


First released in Issue 3. Included for alignment with the POSIX.1-1988 standard and the ANSI C standard.

Issue 6

Extensions beyond the ISO C standard are marked.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/58 is applied, updating the RETURN VALUE and ERRORS sections to add the optional [EOVERFLOW] error as a CX extension.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/59 is applied, adding the tzset() function to the SEE ALSO section.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0393 [104] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0228 [724] is applied.

End of informative text.


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