Testing for DST

example.js

var dst = function(){
	var timeZone = 'EDT';
	function dstDetect(dateStamp){
		if (typeof(dateStamp) == 'undefined') {
			var now = new Date();
			dateStamp = now.getTime();
		}
		var timeObject = new Date(dateStamp);
		var month = timeObject.getMonth();
		var months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
		var monthString = months[month];
		var day = timeObject.getDay();
		var days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
		var dayString = days[day];
		var date = timeObject.getDate();
		var dateString = date < 10 ? ('0' + date) : date;
		var rfcDateString =
			dayString + ', ' + dateString + ' '
			+ monthString + ' ' + timeObject.getFullYear();
		var baselineString = rfcDateString + ' 00:00:00 ' + timeZone;
		var dateObject = new Date(baselineString);
		var dateTimeStamp = dateObject.getTime();
		var diffs = [];
		for (var h = 0; h < 24; h++){
			var offsetTime = new Date(dateTimeStamp + (h * 3600000));
			var offsetHour = offsetTime.getHours();
			diffs[h] = offsetHour - h;
		}
		return diffs;
	}

	function dstForward(current, dateStamp)
	{
		if (typeof(current) == 'undefined') {
			current = getCurrentStamp();
		}
		var hourIndex = Math.floor(current / 3600000);
		var dstMap = dstDetect(dateStamp);
		return (dstMap[hourIndex] == 0 && dstMap[0] != 0);
	}

	function dstBack(current, dateStamp)
	{
		if (typeof(current) == 'undefined') {
			current = getCurrentStamp();
		}
		var hourIndex = Math.floor(current / 3600000);
		var dstMap = dstDetect(dateStamp);
		return (dstMap[hourIndex] != 0 && dstMap[0] == 0);
	}
	
	function getCurrentStamp()
	{
		var now = new Date();
		var startDay = new Date(now.getFullYear(), now.getMonth(), now.getDate());
		return now.getTime() - startDay.getTime();
	}
	
	return {
		dstForward : dstForward,
		dstBack : dstBack,
		setTimeZone : function(z){timeZone=z;}
	};
}();