# 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

Construct a local time explicitly.

Run this code

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

    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)

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