PrimeNG calendar form group validation issue

I'm trying to add validation to my form.

discount field cannot be empty and value range needs to be within 0 to 100, time_from and time_to cannot be empty. I cannot fire validation proccess on time_from and time_to. I use PrimeNG Calendar component. I found out that p-calendar validation works great with ngModule, but I couldnt find any solution for form groups.

Component (simplified)

ngOnInit() {
    this.buildForm();
  }

  buildForm(): void {
    this.discountFG = this.fb.group({
      discount: new FormControl('', [Validators.required, CustomValidators.range([0, 100])]),
      time_from: new FormControl('', Validators.required),
      time_to: new FormControl('', Validators.required)
    });

    this.discountFG.valueChanges
      .subscribe(data => this.onValueChanged(data));
  }

  onValueChanged(data?: any) {
    if (!this.discountFG) { return; }
    const form = this.discountFG;

    for (const field in this.formErrors) {
      // clear previous error message (if any)
      this.formErrors[field] = '';
      const control = form.get(field);

      if (control && control.dirty && !control.valid) {
        const messages = this.validationMessages[field];
        for (const key in control.errors) {
          this.formErrors[field] += messages[key] + ' ';
        }
      }
    }
  }

Template (simplified)

          <p-calendar formControlname="time_from" [locale]="pl" dateFormat="yy-mm-dd" [monthNavigator]="true" [yearNavigator]="true"
            yearRange="2010:2030" (blur)="setTimeFrom($event)" readonlyInput="true" required></p-calendar>

          <p-calendar formControlname="time_to" [locale]="pl" dateFormat="yy-mm-dd" [monthNavigator]="true" [yearNavigator]="true"
            yearRange="2010:2030" [minDate]="minDate" readonlyInput="true" required></p-calendar>

Current behavior

Validators dont notice if date is picked and because of this, no event is fired to catch value change, which means onValueChanged thinks time_from and time_to is untouched.

How can I fix this ?