mktime---convert time to arithmetic representation#include <time.h> time_t mktime(struct tm *timp);Description
mktime assumes the time at timp is a local time, and converts
its representation from the traditional representation defined by
struct tm into a representation suitable for arithmetic.
localtime is the inverse of mktime.
Returns
If the contents of the structure at timp do not form a valid
calendar time representation, the result is -1. Otherwise, the
result is the time, converted to a time_t value.
Portability
ANSI C requires mktime.
mktime requires no supporting OS subroutines.