// set up
let start = moment("2018-05-16 12:00:00"); // some random moment in time (in ms)
let end = moment("2018-05-16 12:22:00"); // some random moment after start (in ms)
let diff = end.diff(start);

// execution
let f = moment.utc(diff).format("HH:mm:ss.SSS");
alert(f);