countdown.js

window.Timer = {
  countdown: function(minutes,seconds){
    var currentTime = new Date().getTime();
    Timer.finishTime = currentTime + (minutes * 60 * 1000) + (seconds * 1000);

    Timer.running = setInterval("Timer.ended()",1000)
  },
  ended: function(){
    var currentTime = new Date().getTime();
    console.log('Now: ' + currentTime + '  - ends: ' + Timer.finishTime);

    if(currentTime >= Timer.finishTime){
      alert('arihoo');
      clearInterval(Timer.running);
    }
  }
}

Timer.countdown(00,15)