title: angular 6 input type=date binding result into string
body: <p>I have a date input control</p>
  
  <pre><code>&lt;input placeholder="Start" matInput ([ngModel])="data.startDate" type="date"&gt; &lt;/mat-form-field&gt;
  </code></pre>
  
  <p>The input control shows the material time picker. But when I change the date then the model is updated with a string value</p>
  
  <p>model:</p>
  
  <pre><code>export interface IDate {
      rate?: number | null;
      startDate?: Date | null;
  }
  </code></pre>