This countdown timer takes a boolean observable input stream and counts down to zero. Pushing a true / false through the input stream will reset the count.

rxjs.countdown.ts

const countdown: Rx.Observable = (
  inputStream: Rx.Observable<boolean>,
  duration: number,
  interval: number = 1000
): Rx.Observable<number> => {
  return inputStream
  .switchMap((enabled) => {
    return enabled ? Rx.Observable.timer(0, interval)
       .takeUntil(Rx.Observable.timer(duration + interval)) :
      Rx.Observable.never();
  })
  .map((value) => duration - value * interval);
}

Check out the demo here: https://jsbin.com/sibacav/edit?js,output