function countDown(duration, display) {
      var M = 60;
      var H = 60 * M;
      var D = 24 * H;

      var start = Date.now(),
          diff,
          minutes,
          seconds;

      function timer() {
          // get the number of seconds that have elapsed since
          // startTimer() was called
          diff = duration - (((Date.now() - start) / 1000) | 0);

          // does the same job as parseInt truncates the float
          days    = (diff / D) | 0;
          hours   = ((diff % D) / H) | 0;
          minutes = ((diff % H) / M) | 0;
          seconds = (diff % M) | 0;

          display({
            d: days,
            h: hours,
            m: minutes,
            s: seconds
          });

          if (diff <= 0) {
              // add one second so that the count down starts at the full duration
              // example 05:00 not 04:59
              start = Date.now() + 1000;
          }
      };
      // we don't want to wait a full second before the timer starts
      timer();
      setInterval(timer, 1000);
  }