Javasacript Countdown timer in Days, Hours, Minute, Seconds

I finally got back to looking at this and re-wrote the code and this works like a charm.

var upgradeTime = 172801;
var seconds = upgradeTime;
function timer() {
  var days        = Math.floor(seconds/24/60/60);
  var hoursLeft   = Math.floor((seconds) - (days*86400));
  var hours       = Math.floor(hoursLeft/3600);
  var minutesLeft = Math.floor((hoursLeft) - (hours*3600));
  var minutes     = Math.floor(minutesLeft/60);
  var remainingSeconds = seconds % 60;
  function pad(n) {
    return (n < 10 ? "0" + n : n);
  }
  document.getElementById('countdown').innerHTML = pad(days) + ":" + pad(hours) + ":" + pad(minutes) + ":" + pad(remainingSeconds);
  if (seconds == 0) {
    clearInterval(countdownTimer);
    document.getElementById('countdown').innerHTML = "Completed";
  } else {
    seconds--;
  }
}
var countdownTimer = setInterval('timer()', 1000);
<span id="countdown" class="timer"></span>

Personally I would use the jquery countdown timer integration. It is simple and having number of options to display in different formats. Since I am fairly new to JS as well, this was the easiest way I found to get a count/timer.

http://keith-wood.name/countdown.html