Add days to sharepoint datetime fiels using javascript

var dateString1=$("input[title='Start Date']").attr('value');


 var arrStartDate = dateString1.split("/");
 var date=arrStartDate[1]+"/"+arrStartDate[0]+"/"+arrStartDate[2]

   var startDate = new Date(Date.parse(date));
    alert('start: ' + startDate);
    var expDate = startDate;
    expDate.setDate(startDate.getDate() + count);
    // alert('<br>expire: ' + expDate);

//Month will always return month-1 so add 1 to month
    var mm = expDate.getMonth()+1;
    var dd = expDate.getDate();
    var yyyy = expDate.getFullYear();
    var finaldate = dd + '/' + mm + '/' + yyyy;

  $("input[title='Due Date']").attr('value',finaldate);