Super countdown with jQuery.

countdown.js

var countDown = function (endDate, el) {
  el = el || $('body');
  var leadingZero = function (nb) {
    return (nb < 10) ? "0" + nb : + nb;
  };
  var updateTimer = function (seconds) {
    var days = Math.floor(seconds / 86400);
    seconds -= days * 86400;
    var hours = Math.floor(seconds / 3600);
    seconds -= hours * (3600);
    var minutes = Math.floor(seconds / 60);
    seconds -= minutes * (60);
    return ((days > 0) ? days + " days " : "") + leadingZero(hours) + ":" + leadingZero(minutes) + ":" + leadingZero(seconds);
  };
  var tick = function () {
    // Get seconds
    var nbSeconds = parseInt((+endDate - +new Date) / 1000);
    if (nbSeconds < 1) {
      el.html('Time finished!');
      return;
    }
    el.html(updateTimer(nbSeconds));
    setTimeout(tick, 500);
  };
  tick();
};

Use cases:

countDown(new Date('2013-01-17 02:00:00')); will write the countdown in the body;

countDown(new Date('2013-01-17 02:00:00'), $('#countdown')); will write the countdown in the #countdown element.

Result:
1 days 23:59:38