do_sub_field(a_time, hours, extra, 24, sub_days);
do_sub_time_field(a_time, hours, extra, 24, sub_days);
sub_hours(ha_time_t *a_time, int extra) {	if(extra < 0) {		add_hours(a_time, -extra);	} else {		do_sub_field(a_time, hours, extra, 24, sub_days);	}}