JSFiddle - React, Tailwind, and code Playground

HTML

<link href="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/2.1.1/fullcalendar.min.css" rel="stylesheet" />
<link href="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/2.1.1/fullcalendar.print.css" rel="stylesheet" media="print" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.8.3/moment.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/2.1.1/fullcalendar.min.js"></script>
<div id="calendar"></div>

JavaScript

$('#calendar').fullCalendar({
    selectable: true,
    select: function (start, end, allDay) {
        start = $.fullCalendar.formatRange(start, start, 'YYYY-MM-DD');
        alert('Valor da data: ' + start);
    }
});