JS code to get DST start date/time

testTZ.js

	function testTZ() {
		//we assume here jan1 is not on the "DST edge"
		var jan1 = new Date(new Date().getFullYear(), 0, 1, 0, 0, 0, 0)
			, offset = -jan1.getTimezoneOffset()
			, dstPresent = false, dstMonth = 0, MINUTE_STEP = 5;
		for (var i = 1; i < 12; ++i) {
			var d = new Date(jan1.getFullYear(), i, 1, 0, 0, 0, 0), curOffset = -d.getTimezoneOffset();
			if (curOffset != offset) {
				dstPresent = true;
			}
			if (curOffset < offset) {
				offset = curOffset; //Southern Hemisphere DST end is earlier than DST start
				continue;
			}
			if (curOffset > offset) {
				dstMonth = i - 1;
				break;
			}
		}
		document.write("dst ? " + dstPresent + ", month == " + dstMonth);
		if (dstPresent) {
			var dstDay = 1, dstHour = 0, dstMinute = 0
				, day1 = new Date(jan1.getFullYear(), dstMonth, dstDay, dstHour, dstMinute, 0, 0)
				, offset = -day1.getTimezoneOffset();
			for (var i = 2; i < 32; ++i) {
				var d = new Date(day1.getFullYear(), dstMonth, i, dstHour, dstMinute, 0, 0), curOffset = -d.getTimezoneOffset();
				if (d.getMonth() != dstMonth) {
					break; //move out of month, most probably impossible TODO check
				}
				if (curOffset > offset) {
					dstDay = i - 1;
					break;
				}
			}
			day1 = new Date(jan1.getFullYear(), dstMonth, dstDay, dstHour, dstMinute, 0, 0);
			for (var i = 1; i < 24; ++i) {
				var d = new Date(day1.getFullYear(), dstMonth, dstDay, i, dstMinute, 0, 0), curOffset = -d.getTimezoneOffset();
				if (curOffset > offset) {
					dstHour = i - 1;
					break;
				}
			}
			day1 = new Date(jan1.getFullYear(), dstMonth, dstDay, dstHour, dstMinute, 0, 0);
			for (var i = MINUTE_STEP; i < 60; i = i + MINUTE_STEP) {
				var d = new Date(day1.getFullYear(), dstMonth, dstDay, dstHour, i, 0, 0), curOffset = -d.getTimezoneOffset();
				if (curOffset > offset) {
					dstMinute = i - MINUTE_STEP; //New Zealand - Chatham Islands has "DST minute" == 45
					break;
				}
			}
			document.write("; dstDay == " + dstDay + "; dstHour == " + dstHour + "; dstMinute == " + dstMinute);
		}
	}