title: Ionic 4 datetime picker does not update its variable
body: <p>I am using the ionic 4 <code>ion-datetime</code> picker to try to select a date in a form, though when a new date is selected, it does not update the variable.</p>
  
  <p>I've tried using all of the different combinations I know, but I've included all of them here in this example.</p>
  
  <pre><code>&lt;form [formGroup]="upsert" (ngSubmit)="submitUpsert()"&gt;
    &lt;ion-datetime
      display-format="MM DD, YYYY"
      picker-format="MM DD YYYY"
      min="2010"
      max="2030"
      class="form__group__item--input has-value"
      formControlName="date"
      placeholder="Select Date"
      [value]=date
      [(ngModel)]="event.today"
      (ngModelChange)="event.today"
    &gt;&lt;/ion-datetime&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>and in my form controller I have:</p>
  
  <pre><code>constructor(
    private db: DatabaseService,
    private modalController: ModalController,
    private formBuilder: FormBuilder
  ) {
    this.upsert = this.formBuilder.group({
      title: ['', Validators.compose([Validators.required])],
      date: ['', Validators.compose([Validators.required])],
      notes: [''],
      location: [''],
      timezone: [''],
    });
    this.event.today = moment().toISOString(true);
    this.event.timezone = moment().format('ZZ');
  }
  </code></pre>
  
  <p>And eventually, I use the form submit action</p>
  
  <pre><code>submitUpsert() {
    console.log("new date", this.event.today);
    if(this.upsert.invalid) {
      const error = {
        code: "001",
        message: this.upsert.status
      };
      this.showError(error);
    } else {
      this.db.upsert(this.upsert.value).then(eid =&gt; {
        console.log("Success", eid);
        this.hideUpsert();
      }, error =&gt; {
        this.showError(error);
        console.error(error);
      });
    }
  }
  </code></pre>
  
  <p>In the latest beta API, it does not mention to use <code>[(ngModel)]</code> or <code>(ngModelChange)</code>, but it was the only way I could get a default date (of today) pre-selected.  </p>
  
  <p>Unfortunately, it's not updating the model nor the form object.  Any thoughts?</p>