//USES MASTER PAGE
  var startTime =  document.getElementById('<%=txtstartDate.ClientID%>').value;//START DATE
            var objvalDur = document.getElementById('<%=ddlDuration.ClientID%>').value;//duration

//calculate end date
   var d = new Date(startTime);
                    d.setDate(d.getDate()+durTime);
                    var exactDate = (d.getMonth()+1)+"/"+d.getDate()+"/"+d.getYear();
                    document.getElementById('<%=txtendDate.ClientID%>').value=exactDate;