How to add Month in given date using Jscript.

 var startdate=Xrm.Page.data.entity.attributes.get(startingdate).getValue();
 var duration=Xrm.Page.data.entity.attributes.get(nodays).getValue();
  var endwarrantydate=Xrm.Page.data.entity.attributes.get(endingdate);
  if(startdate!=null && duration!=null)
  {            
var enddate= startdate.setMonth(startdate.getMonth() + duration);
        endwarrantydate.setValue(enddate);
  }