NAME

std::mktime - std::mktime

Synopsis

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