get_switching_time
zone_start = get_switching_time(tmp.tm_isdst, t);
t = get_switching_time(tmp1.tm_isdst,
t = get_switching_time(tmp1.tm_isdst,
zone_start = get_switching_time(tmp2.tm_isdst,
static time_t get_switching_time(int, time_t);