JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.2/moment.min.js"></script>
<input type="hidden" id="dtconclusao" value="25/09/2018" />
<input type="hidden" id="dtfim" value="30/09/2018" />

JavaScript

var dtconclusao = document.getElementById("dtconclusao").value;
var dtfim = document.getElementById("dtfim").value;
var data1 = moment(dtconclusao);
var data2 = moment(dtfim);
var diferenca = data1.diff(data2, 'dias');

alert(diferenca);