Just wanted to put my 2 cents in. I modified @Ajay Singh's function to handle countdown and count up Here is a snip from the jsfiddle.
var countDown = Math.floor(Date.now() / 1000)
runClock(null, function(e, r){ console.log( e.seconds );}, countDown);
var t = setInterval(function(){
runClock(function(){
console.log('done');
clearInterval(t);
},function(timeElapsed, timeRemaining){
console.log( timeElapsed.seconds );
}, countDown);
}, 100);