Angular not detecting change to first element in array

I'm trying to display a list of dates for the current week in an Angular app. I want to allow users to view previous weeks at the click of a button, so I'm using an Observable to update the array of dates, and attempting to display the updated array.

All items are updated in the view, except for the first item in the array. Plunker example here

I've tried using *ngFor and the async pipe, as well as explicitly creating elements for each item in the array (like below). Both have the same issue. I'm struggling to find a solution.

//our root app component
import {Component, NgModule, VERSION} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import 'rxjs/Observable';
import { Subject } from 'rxjs/Subject';

@Component({
  selector: 'my-app',
  template: `
    <button (click)="previousWeek()">Prev Week</button>
    <div>{{dates[0]}}</div>
    <div>{{dates[1]}}</div>
    <div>{{dates[2]}}</div>
  `,
})
export class App {
  name:string;
  dates: Date[];
  public $datesSource: Observable<Date[]>;
  private datesSource: Subject<Date[]>;

  constructor() {
    this.datesSource = new Subject<Date[]>();
    this.datesSource$ = this.getDatesWithObservable();
    this.datesSource$.subscribe((dates) => {
      console.log(dates);

      this.dates = dates;
    })

    this.setDates(new Date());
  }

  setMonday(date: Date): Date {
        const day = date.getDay() || 7;
        if (day !== 1) {
            date.setHours(-24 * (day - 1));
        }
        return date;
    }

  setDates(date: Date): void {
        const dates = [
            new Date(),
            new Date(),
            new Date(),
            new Date(),
            new Date(),
            new Date(),
            new Date()
        ];
        const monday = this.setMonday(date);
        dates[0] = monday;
        const mondayDate = monday.getTime();
        dates.forEach((date, idx) => {
            console.log(idx);

            date.setTime(monday.getTime() + (idx * 24 * 60 * 60 * 1000));
    });
        this.addDates(dates);
    }

    addDates(dates: Date[]): void {
        this.datesSource.next(dates);
    }

    getDatesWithObservable(): Observable<Date[]> {
        return this.datesSource.asObservable();
    }


    previousWeek(): void {
      const day = this.dates[0].getDay() || 7;
      const lastWeek = this.dates[0];
      const days = 7;
      lastWeek.setTime(lastWeek.getTime() - (days * 24 * 60 * 60 * 1000));
      this.setDates(lastWeek);
    }
}