title: MAT_DATE_FORMATS definition/meaning of fields
body: <p>Material example for <a href="https://material.angular.io/components/datepicker/overview#customizing-the-parse-and-display-formats" rel="noreferrer">customizing-the-parse-and-display-formats</a> for date picker uses custom MAT_DATE_FORMATS  </p>
  
  <pre><code>export const MY_FORMATS = {
    parse: {
      dateInput: 'LL',
    },
    display: {
      dateInput: 'LL',
      monthYearLabel: 'MMM YYYY',
      dateA11yLabel: 'LL',
      monthYearA11yLabel: 'MMMM YYYY',
    },
  };
  </code></pre>
  
  <p>I could not find where and how these fields like <code>dateA11yLabel</code> will come into play. What I could figure out was <code>display.dateInput</code> is used in displaying selected date on calendar and <code>display.monthYearLabel</code> is used in the select drop-down of year selector.</p>
  
  <ol>
  <li>Where are rest of the fields used?</li>
  <li>When writing custom MAT_DATE_FORMATS is it mandatory to define all
  fields?</li>
  </ol>