how can add days in date in javascript or jquery

Question

  <script type ="text/javascript">
            function change_date() {
                var now = new Date(document.getElementById("<%= txt_start_date.ClientID%>").value);
                var add_day = new Date(document.getElementById("<%= txt_no_days.ClientID%>"));
                document.getElementById("<%= txt_end_date.ClientID%>").value = now.toDateString(c.getDate + add_day);
            }
             </script>
this code is not working pls give any suggesion