var _MS_PER_DAY = 1000 * 60 * 60 * 24;

  // a and b are javascript Date objects
  function dateDiffInDays(a, b) {
    // Discard the time and time-zone information.
    var utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
    var utc2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());

    return Math.floor((utc2 - utc1) / _MS_PER_DAY);
  }

  console.log(dateDiffInDays(new Date("12/2/2012"), new Date()) + 1);