Moment.js Sandbox

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.2.1/moment.min.js"></script>

JavaScript

const startValue = "1:12";
const stopValue = "2:10"; // for a duration of 58 minutes
const start = { hour: startValue.split(":")[0], minute: startValue.split(":")[0] }
const stop = { hour: stopValue .split(":")[0], minute: stopValue .split(":")[0] }

const msDuration = moment().hours(stop.hour).minutes(stop.minute).milliseconds(0).valueOf() - moment().hours(start.hour).minutes(start.minute).milliseconds(0).valueOf();

console.log("msDuration: ", msDuration);