JavaScript で日付に月を正確に加算するにはどうすればよいですか?

<code class="js">function addMonths(dateObj, num) {
  return dateObj.setMonth(dateObj.getMonth() + num);
}</code>

<code class="js">function addMonths(dateObj, num) {
  var currentMonth = dateObj.getMonth() + dateObj.getFullYear() * 12;
  dateObj.setMonth(dateObj.getMonth() + num);
  var diff = dateObj.getMonth() + dateObj.getFullYear() * 12 - currentMonth;

  if (diff != num) {
    dateObj.setDate(0);
  }

  return dateObj;
}</code>

<code class="js">function addMonths(date, months) {
  var d = date.getDate();
  date.setMonth(date.getMonth() + +months);
  if (date.getDate() != d) {
    date.setDate(0);
  }
  return date;
}</code>