mergeObservables.ts

import { Component } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Subject } from 'rxjs/Subject';
import 'rxjs/add/observable/interval';
import 'rxjs/add/operator/map';
import 'rxjs/add/observable/merge';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  clock: Observable<any>;

  clock$ = new Subject();

  constructor() {
    this.clock = Observable.merge(this.clock$,
      Observable.interval(3000))
      .map(() => new Date());
  }
}
    
    
 // html part    
{{ clock | async | date: 'medium' }}