date manip

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.13.0/moment.min.js"></script>
Start: 
<input id="startDate" type="date" value="2016-05-01"> + 
<input id="nbDays" value="2" size="3"> days 
<input value="=" id="go" type="button">
<input id="endDate">

JavaScript

$('#go').on('click', function() {

  var nbdays_string = $("#nbDays").val();
  var startDate_string = $("#startDate").val();
  
  var nbdays  = parseFloat( nbdays_string ) 
  var startDate = moment(startDate_string);
  
  var endDate = startDate.add( nbdays, 'days').format('DD/MM/YYYY');

  $("#endDate").val(endDate);
});