rxjs-observable-merge-startwith-scan

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';
import 'rxjs/add/operator/startWith';
import 'rxjs/add/operator/scan';

@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))
      .startWith(new Date())
      .scan((accu: Date, current) => {
          const date = new Date(accu.getTime());
          date.setSeconds(date.getSeconds() + 1);
          return date;
      });
  }
}