codice:
      <script type="text/javascript">
      <!--

        var _oldDate = "19/02/1986"; 	/* original date in format dd/mm/yyyy */
        var _addDays = 30;		/* days to add to the date */

        /*************************************************************************************************/
        var _newDate = null;
        var _tmpDate = null;

       _oldDate = _oldDate.split( '/' );
       _tmpDate = new Date( _oldDate[2], _oldDate[1]-1, _oldDate[0] );    
       _tmpDate = new Date( _tmpDate.getTime( ) + _addDays*24*60*60*1000 );

       _newDate = ( _tmpDate.getDate() > 10 ? _tmpDate.getDate() : "0" +_tmpDate.getDate() ) + '/' +
                  ( _tmpDate.getMonth()+1 > 10 ? _tmpDate.getMonth()+1 : "0" + (_tmpDate.getMonth()+1) ) + 
                  '/' + _tmpDate.getFullYear();
        /*************************************************************************************************/

        alert( _newDate );		/* '_newDate' contains the modified date */ 

      //-->
      </script>