# mktime

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< c‎ | chrono

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

### See also

localtime localtime_s (C11) | converts time since epoch to calendar time expressed as local time (function)

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