Try jQuery:
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.4/themes/smoothness/jquery-ui.css" />
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.4/jquery-ui.min.js"></script></script>
<script>
$(document).ready(function(){
$( "#datepicker1" ).datepicker({
onClose: function() {
var date2 = $('#datepicker1').datepicker('getDate');
date2.setDate(date2.getDate()+3)
$( "#datepicker2" ).datepicker("setDate", date2);
}
});
$( "#datepicker2" ).datepicker();
});
</script>
</head>
<body>
<p>Date1: <input type="text" id="datepicker1"></p>
<p>Date2: <input type="text" id="datepicker2"></p>
</body>
</html>