# 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 std::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

Construct a local time explicitly.

Run this code

    #include <ctime>
    #include <iomanip>
    #include <iostream>
    #include <sstream>

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

        std::tm tm{}; // Zero initialise
        tm.tm_year = 2020 - 1900; // 2020
        tm.tm_mon = 2 - 1; // February
        tm.tm_mday = 15; // 15th
        tm.tm_hour = 10;
        tm.tm_min = 15;
        tm.tm_isdst = 0; // Not daylight saving
        std::time_t t = std::mktime(&tm);
        std::tm local = *std::localtime(&t);

        std::cout << "local: " << std::put_time(&local, "%c %Z") << '\n';
    }

Possible output:

    local: Sat Feb 15 10:15:00 2020 PST

### See also

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

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