how to get date difference in javascript

<script>

function calcDate() {
var today = new Date();
var past = new Date(2010,05,01);
    var diff = Math.floor(today.getTime() - past.getTime());
    var day = 1000 * 60 * 60 * 24;

    var days = Math.floor(diff/day);
    var months = Math.floor(days/31);
    var years = Math.floor(months/12);

    var message = past.toDateString();
    message += " was "
    message += days + " days "
    message += months + " months "
    message += years + " years ago \n"

  alert(message);
  return false;
    }
</script>