countdown.js

 var countdown=function(end, elements, callback){
	var _second = 1000,
	    _minute = _second*60,
		_hour = _minute*60,
		_day = _hour * 24,
		
		end = new Date(end),
		timer,
		calculate = function (){
			
			var now=new Date(),
				remaining= end.getTime() - now.getTime(),
				data;
			
			if(isNaN(end)){
				console.log('Invalid date/time');
				return;
			}
			
			if(remaining<=0){
				clearInterval(timer);
				
				if(typeof callback === 'function'){
					callback();
				}	
				
			}else {
				if(!timer){
					timer=setInterval(calculate, _second);
				}
				
				data={
				'days': Math.floor(remaining / _day),
				'hours': Math.floor((remaining % _day) / _hour),
				'minutes': Math.floor((remaining % _hour) / _minute),
				'seconds': Math.floor((remaining % _minute) / _second)
				}

				if(elements.length){
					for(x in elements){
						var x = elements[x];
						data[x]= ('00' + data[x]).slice(-2);
						document.getElementById(x).innerHTML = data[x];
					}
				}
				
				//console.log(data);
			}
				
		};
	
	calculate();
		 
 }