JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/themes/cupertino/jquery-ui.css">
Start: <input type="text" id="startdate" class="datepicker"><br />
<input type="hidden" id="numofdays" value="365"><br />
Calc: <input type="text" id="calc">

JavaScript

$("#startdate").datepicker({
    dateFormat: 'd/m/y',
    onSelect: function(dateStr) {
        var nights = parseInt($('#numofdays').val());
        var depart = $.datepicker.parseDate('d/m/y', dateStr);
        alert(depart);
        depart.setDate(depart.getDate('d/m/y') + nights);
        $('#calc').val(depart.toLocaleDateString());
    }
});