example-4.ts

declare const stringDateValidator: ValidatorFn;
declare const stringToDate: (value: string) => Date | null;
declare const dateToString: (value: Date | null) => string;

class ExampleFourComponent implements OnInit {
  inputControl = new FormControl('', {
    validators: stringDateValidator,
  });

  dateControl = new FormControl<Date | null>(null);

  ngOnInit() {
    this.inputControl.events
      .pipe(
        map(event => {
          if (event.type === 'StateChange' && event.changes.has('value')) {
            const changes = new Map(event.changes);

            changes.set('value', stringToDate(changes.get('value')));

            return {
              ...event,
              changes,
            };
          }

          return event;
        }),
      )
      .subscribe(this.dateControl.source);

    this.dateControl.events
      .pipe(
        map(event => {
          if (event.type === 'StateChange' && event.changes.has('value')) {
            const changes = new Map(event.changes);

            changes.set('value', dateToString(changes.get('value')));

            return {
              ...event,
              changes,
            };
          }

          return event;
        }),
      )
      .subscribe(this.inputControl.source);
  }
}