mktime
Syntax:
#include <ctime> time_t mktime( struct tm *time );
The mktime function converts the local time in time to calendar time, and returns it.
The elements tm_wday and tm_yday of the struct time are recalculated and reset based on the other elements of the struct.
If there is an error, -1 is returned and tm_yday and tm_wday remain unchanged.