JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://momentjs.com/downloads/moment.min.js"></script>

JavaScript

var start_time = "12:00";
    var end_time = "16:00";

    var start = moment.duration(start_time, 'h');
    var end = moment.duration(end_time, 'h');
    alert("moment: "+end.subtract(start).hours()); // 4


    alert("pure js: "+(parseInt(end_time, 10)-parseInt(start_time, 10)));