JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.js"></script>
Select the radio first<br /><br />
<input type="radio" id="radio1" name="length" value="4"/><label for="radio1">4 DAYS</label>
<input type="radio" id="radio2" name="length" value="7"/><label for="radio2">7 DAYS</label>
<input type="radio" id="radio3" name="length" value="14"/><label for="radio3">14 DAYS</label>
<input type="radio" id="radio4" name="length" value="30"/><label for="radio4">30 DAYS</label>
<br /><br />
Then input Arrive date
<br /><br />
Arrive: <input type="text" id="arrivalDate" class="datepicker"><br />
Depart: <input type="text" id="departureDate">

JavaScript

$('#arrivalDate').datepicker({
        onSelect: function(dateStr) {
            var depart = $.datepicker.parseDate('mm/dd/yy', dateStr);
            depart.setDate(depart.getDate() + $('input:radio[name=length]:checked').val());
            $('#departureDate').val (depart);
         }
    });