mktime

time_t mktime( struct tm *time );

Renormalizes local calendar time expressed as a struct tm object and also converts it to time since epoch as a time_t object. time->tm_wday and time->tm_yday are ignored. The values in time are not checked for being out of range.

A negative value of time->tm_isdst causes mktime to attempt to determine if Daylight Saving Time was in effect in the specified time.

If the conversion to time_t is successful, the time object is modified. All fields of time are updated to fit their proper ranges. time->tm_wday and time->tm_yday are recalculated using information available in other fields.

Parameters

Return value

time since epoch as a time_t object on success or -1 if time cannot be represented as a time_t object (POSIX also requires EOVERFLOW to be stored in errno in this case).

Notes

If the struct tm object was obtained from POSIX strptime or equivalent function, the value of tm_isdst is indeterminate, and needs to be set explicitly before calling mktime.

Example

#include <stdio.h>
#include <time.h>
 
int main(void)
{
    struct tm tm = *localtime(&(time_t){time(NULL)});
    printf("Today is           %s", asctime(&tm));
    tm.tm_mon -= 100;  // tm_mon is now outside its normal range
    mktime(&tm); // recalculate tm
    printf("100 months ago was %s", asctime(&tm));
}

Today is           Tue Feb 17 13:46:01 2015
100 months ago was Tue Oct 17 14:46:01 2006

References

• C11 standard (ISO/IEC 9899:2011):
  • 7.27.2.3 The mktime function (p: 390-391)
• C99 standard (ISO/IEC 9899:1999):
  • 7.23.2.3 The mktime function (p: 340-341)
• C89/C90 standard (ISO/IEC 9899:1990):
  • 4.12.2.3 The mktime function

See also