mktime - cppreference.com (original) (raw)

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 are not checked for being out of range.

A negative value of arg->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 arg object is modified. All fields of arg are updated to fit their proper ranges. arg->tm_wday and arg->tm_yday are recalculated using information available in other fields.

[edit] Parameters

arg	-	pointer to a tm object specifying local calendar time to convert

[edit] Return value

The 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 in this case).

[edit] 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.

[edit] 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"); }

Possible output:

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)

[edit] References