JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="css/smoothness/jquery-ui-1.8.14.custom.css">
<form name="editfrm" id="editfrm">
Data: <input type="text" id="arrivalDate" class="datepicker"><br />
Dias: <input type="text" id="numOfNights" value="0"><br />

Resultado: <input type="text" id="departureDate">
</form>

JavaScript

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

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

            
});
});