JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.js"></script>
Arrive: <input type="text" id="arrivalDate" class="datepicker"><br />
Nights: <input type="text" id="numOfNights" value="0"><br />
Depart: <input type="text" id="departureDate">

JavaScript

$('#arrivalDate').datepicker({
        onSelect: function(dateStr) {
            var nights = parseInt($('#numOfNights').val());
            var depart = $.datepicker.parseDate('mm/dd/yy', dateStr);
            depart.setDate(depart.getDate() + nights);
            $('#departureDate').val(depart);
         }
    });
    
$('#numOfNights').change(function() {
            var nights = parseInt($('#numOfNights').val());
            var depart = $.datepicker.parseDate('mm/dd/yy', $('#arrivalDate').val());
            depart.setDate(depart.getDate() + nights);
            $('#departureDate').val(depart);
});