Edit in JSFiddle

/* Timer */
function countdown( elementId, seconds ){
  var element, endTime, hours, mins, msLeft, time;

  function updateTimer(){
    msLeft = endTime - (+new Date);
    if ( msLeft < 0 ) {
      console.log('done');
    } else {
      time = new Date( msLeft );
      hours = time.getUTCHours();
      mins = time.getUTCMinutes();
      element.innerHTML = (hours ? hours + ':' + ('0' + mins).slice(-2) : mins) + ':' + ('0' + time.getUTCSeconds()).slice(-2);
      setTimeout( updateTimer, time.getUTCMilliseconds());
      // if you want this to work when you unfocus the tab and refocus or after you sleep your computer and come back, you need to bind updateTimer to a $(window).focus event^^
    }
  }

  element = document.getElementById( elementId );
  endTime = (+new Date) + 1000 * seconds;
  updateTimer();
}

countdown('countdown', 43200);	 // second base

<div id="countdown"></div>

div {
  color: #1B232F;
  font-family: Verdana, Arial, sans-serif;
  font-size: 18px;
  font-weight: bold;
  text-decoration: none;
}