Find the time zone

timezone.c

#include <time.h>
signed long getTZ ()
{
    struct tm* timeStruct;
    time_t timeValBase = 86400; /* I know an implementation in which time_t is unsigned */
    time_t timeVal;

    timeStruct = gmtime(&timeValBase); /* parse as the universal time */
    timeVal = mktime(tp); /* intentionally treat as the local time in order to find the time zone */

    return (signed long)timeValBase - (signed long)timeVal; /* time zone in seconds, e.g. 32400 in Japan */
}