How do I get the date that is 7 days from the date that is selected in datepicker?

How do I get the date that is 7 days from the date that is selected in datepicker?

I'm trying to alert the date that is 7 days from when a date is selected from datepicker.  So far I have

code:
  1. $("#start_date").datepicker({
  2.         dateFormat: 'yy-mm-dd',
  3.         onSelect: function(dateText, inst)
  4.         {
  5.             alert($("#start_date").datepicker( "option", "defaultDate", +7));
  6.               
  7.         }
  8. });

That code alerts [object object].  How do I get the date that is 7 days from the date that is selected in datepicker?

-Thank you,
     Rich