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="1">
<br />Depart:
<input type="text" id="departureDate">

JavaScript

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

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