datediff.js

function datediff(aDate1, aDate2) {
  var diff = Math.abs(aDate2 - aDate1);

  var milliseconds = diff % 1000;
  var seconds = Math.floor(diff / 1000 % 60);
  var minutes = Math.floor(diff / (1000 * 60) % 60);
  var hours = Math.floor(diff / (1000 * 60 * 60) % 24);
  var days = Math.floor(diff / (1000 * 60 * 60 * 24) % 365);

  return [
    ("00" + days).slice(-3), //   DDD
    ("0" + hours).slice(-2), //   HH
    ("0" + minutes).slice(-2), // MM
    ("0" + seconds).slice(-2), // SS
  ].join(":");
}