JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.5.1/moment.min.js"></script>
Start Time: <input type="text" id="startTime">
<br>
End time: <input type="text" id="endTime">
<br>
Results (in hours): <input type="text" id="results">
<br>
<br>
<button type="button" id="calculate">calculate</button>

JavaScript

var startTime = $('#startTime').val();
var endTime = $('#endTime');
var hours = moment(endTime, 'YYYY/MM/DD HH:mm').diff(moment(startTime, 'YYYY/MM/DD HH:mm')).duration().asHours();

$(document).on('click', "#calculate", function() {
	$('#results').val(hours);
});