Javascript count down

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Javascript

I try to customize this countdown timer, I want to change days, hours, minutes, seconds for example: 5:16:55:56
Or do you know a simplar way to make a countdown where i can set numbers of days, hours, minutes and seconds as i want?

<span id="countdown" class="timer"></span>
<script>
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);
</script>

What I have tried:

I tried changing the values between brakets