https://jsfiddle.net/ggk5L9of/17/

rxjs-countdown.ts

const source = Rx.Observable.interval(1000);

const startDate = moment().add(1, 'minute');

const stream = Rx.Observable.merge(
  Rx.Observable.of(undefined),
  Rx.Observable.interval(10000)
).map(i => startDate);

Rx.Observable
  .combineLatest(source, stream)
  .map(([counter, value]) => {
  	return moment(value).diff(moment());
  })
  .distinctUntilChanged()
  .filter(value => value > 0)
  .subscribe(value => {
	  console.log(moment.utc(value).format('HH:mm:ss'));
  });