# std::mktime

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< cpp‎ | chrono‎ | c

Defined in header `<ctime>` |  |
std::time_t mktime( std::tm* time ); |  |

Converts local calendar time to a time since epoch as a time_t object. `time->tm_wday` and `time->tm_yday` are ignored. The values in `time` are permitted to be outside their normal ranges.

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

If the conversion 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 std::tm object specifying local calendar time to convert

### Return value

Time since epoch as a std::time_t object on success or -1 if `time` cannot be represented as a std::time_t object.

### Notes

If the std::tm object was obtained from std::get_time or the POSIX strptime, the value of `tm_isdst` is indeterminate, and needs to be set explicitly before calling `mktime`.

### Example

Display the time 100 months ago.

Run this code

    #include <iostream>
    #include <iomanip>
    #include <ctime>
    #include <stdlib.h>

    int main()
    {
        setenv("TZ", "/usr/share/zoneinfo/America/New_York", 1); // POSIX-specific

        std::time_t t = std::time(NULL);
        std::tm tm = *std::localtime(&t);
        std::cout << "Today is           " << std::put_time(&tm, "%c %Z")
                  << " and DST is " << (tm.tm_isdst ? "in effect" : "not in effect") << '\n';
        tm.tm_mon -= 100;  // tm_mon is now outside its normal range
        std::mktime(&tm);  // tm_dst is not set to -1; today's DST status is used
        std::cout << "100 months ago was " << std::put_time(&tm, "%c %Z")
                  << " and DST was " << (tm.tm_isdst ? "in effect" : "not in effect") << '\n';
    }

Output:

    Today is           Fri Apr 22 11:40:36 2016 EDT and DST is in effect
    100 months ago was Sat Dec 22 10:40:36 2007 EST and DST was not in effect

### See also

localtime | converts time since epoch to calendar time expressed as local time (function)
C documentation for mktime

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