mktime

Defined in header `<time.h>`
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

time

-

pointer to a tm object specifying local calendar time to convert

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

Run this code

#define _POSIX_C_SOURCE 200112L // for setenv on gcc
#include <stdlib.h>
#include <stdio.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_dst 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");
}

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)

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

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications

Functions
Time manipulation
difftime
time
clock
timespec_get (C11)
Format conversions
asctimeasctime_s (C11)
ctimectime_s (C11)
strftime
wcsftime (C95)
gmtimegmtime_s (C11)
localtimelocaltime_s (C11)
mktime
Constants
CLOCKS_PER_SEC
Types
tm
time_t
clock_t
timespec (C11)