Javascript Date ago

Date.prototype.__defineGetter__('ago', function () {
 var diff = (((new Date()).getTime() - this.getTime()) / 1000)
  , day_diff = Math.floor(diff / (60 * 60 * 24));
 return day_diff == 0 && (
  diff < 60 && 'just now' ||
  diff < 120 && '1 minute ago' ||
  diff < 3600 && Math.floor( diff / 60 ) + ' minutes ago' ||
  diff < 7200 && '1 hour ago' ||
  diff < 86400 && Math.floor( diff / 3600 ) + ' hours ago') ||

  day_diff == 1 && 'Yesterday' ||
  day_diff < 7 && day_diff + ' days ago' ||
  Math.ceil( day_diff / 7 ) + ' weeks ago';
});/* w ww  .  j a v a  2  s.c o  m*/
var a = new Date('12/12/1990');
console.log(a.ago);