DDim1000,
возможно в документации есть метод проще
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.17.1/moment.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.17.1/locale/ru.js"></script>
</head>
<body>
<p></p>
<script>
moment.locale('ru');
var UnlockRemindPass = moment().add(2,'hours').valueOf();
function fn()
{
var DateTime = moment().valueOf();
var d = UnlockRemindPass - DateTime;
document.querySelector("p").innerHTML = moment(d).utc().format('HH:mm:ss'); ;
d > 0 && window.setTimeout(fn, 300)
}
fn()
</script>
</body>
</html>