mktime

Renormalizes local calendar time expressed as a struct tm object and also converts it to time since epoch as a time_t object. arg->tm_wday and arg->tm_yday are ignored. The values in arg

A negative value of arg->tm_isdst causes mktime

If the conversion to time_t is successful, the arg object is modified. All fields of arg are updated to fit their proper ranges. arg->tm_wday and arg->tm_yday

Parameters

Return value

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

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

#define _POSIX_C_SOURCE 200112L // for setenv on gcc
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main(void)
{
    setenv("TZ", "/usr/share/zoneinfo/America/New_York", 1); // POSIX-specific
 
    struct tm tm = *localtime(&(time_t){time(NULL)});
    printf("Today is           %s", asctime(&tm));
    printf("(DST is %s)\n", tm.tm_isdst ? "in effect" : "not in effect");
    tm.tm_mon -= 100;  // tm_mon is now outside its normal range
    mktime(&tm);       // tm_isdst is not set to -1; today's DST status is used
    printf("100 months ago was %s", asctime(&tm));
    printf("(DST was %s)\n", tm.tm_isdst ? "in effect" : "not in effect");
}

Today is           Fri Apr 22 11:53:36 2016
(DST is in effect)
100 months ago was Sat Dec 22 10:53:36 2007
(DST was not in effect)

References

See also