dateobj = <input onkeyup="eval('dateobj=new Date('+value+');$(\'output\').innerHTML=dateDiff(dateobj, now);');" value="2013, 2, 31, 20, 0, 0" ><br>
now = <input onkeyup="eval('now=new Date('+value+');$(\'output\').innerHTML=dateDiff(dateobj, now);');">
<div id=output></div>

<script src="/rp/js/lib.js"></script>
<script>
	myPrettyDateObjectInTheFuture = dateobj = new Date(2013, 2, 31, 20, 0, 0)
	myPrettyDateObjectRightNow    = now     = new Date(2013, 2, 30, 20, 0, 0);
	function dateDiff(dateobj, now) {
		// So basically a function that's passed the two date objects created above
		
		// Get the time difference in unix time (in seconds: the /1000 is because JS uses milisecs)
		var unixt = Math.floor((dateobj.getTime() - now.getTime()) / 1000);
		//                 days,      hours, mins, seconds
		var timeperiods = [3600 * 24, 3600,  60,   1];
		var output = "";
		for (var i in timeperiods) {
			var t = Math.floor(unixt / timeperiods[i]);
			if (t < 0)
				t = 0;
			
			output += ":" + leadingZeros(t);
			unixt -= timeperiods[i] * t;
		}
		return output.substr(1);
	}
	
	$("output").innerHTML = dateDiff(dateobj, now);
</script>