#include <time.h>time_t mktime(struct tm *time);
The mktime( ) function returns the calendar-time equivalent of the broken-down time found in the structure pointed to by time. The elements tm_wday and tm_yday are set by the function, so they need not be defined at the time of the call.
If mktime( ) cannot represent the information as a valid calendar time, –1 is returned.
Related functions are time( ), gmtime( ), asctime( ), and ctime( ).