timer.js

var Timer = function(end, options){
  if(!options) options = {};
  if(!options['timerInterval']) options['timerInterval'] = 1;
  if(!options['periods']) options['periods'] = {
    'milliseconds' : 1,
    'seconds' : 1000,
    'minutes' : 60,
    'hours' : 60,
    'days' : 24,
    'weeks' : 7
  }
  if(!end) throw('End date not provided');
  var date = new Date(end);
  var timer = this;
  var timer_id;
  var periods_in_milliseconds = [];
  var m = 1;
  for(period in options.periods){
    periods_in_milliseconds.push([period, m * options.periods[period]]);
    m *= options.periods[period];
  }
  periods_in_milliseconds = periods_in_milliseconds.reverse();
  this.calculate = function(){
   var difference = date.getTime() - (new Date()).getTime();
   if(difference <= 0){
     if(options['onComplete'], options['onComplete'](date));
     return;
   }
   var periods = {}
   for(var i=0;i<periods_in_milliseconds.length;i++){
     var divisor = periods_in_milliseconds[i][1];
     periods[periods_in_milliseconds[i][0]] = Math.floor(difference / divisor);
     difference %= divisor;
   }
   if(options['onTimer']) options['onTimer'](periods);
  }
  this.stop = function(){
    clearTimeout(timer_id);
  }
  this.start = function(){
    timer_id = setInterval(function(){
      timer.calculate();
    }, options['timerInterval']);
    timer.calculate();
  }
};