JSFiddle - React, Tailwind, and code Playground

HTML

<form>

	<input type="date" id="dateini">

	<input type="date" id="datefin">

	<input type="button" value="calcular" onclick="calcular()">


</form>
  <input type="text" id="resultado" readonly>

JavaScript

function calcular(){

	var fechaini = new Date(document.getElementById('dateini').value);

	var fechafin = new Date(document.getElementById('datefin').value);

	var diasdif= fechafin.getTime()-fechaini.getTime();

	var contdias = Math.round(diasdif/(1000*60*60*24));
  
  document.getElementById('resultado').value = contdias;
	//alert(contdias);

}