How do I verify that the form input date is valid?
If the date is valid, then the form state will return an object of type Drupal\Core\Datetime\DrupalDateTime, and otherwise it will return an array with the invalid values. Since the invalid date format will be caught elsewhere, the custom code need only check if the value is an instance of DrupalDateTime:

if ($date_of_meeting instanceof DrupalDateTime) {
  $formatted_date_of_meeting = $date_of_meeting-&gt;format('l, j F Y');
  $node-&gt;setTitle($name_of_meeting.' on '.$formatted_date_of_meeting);
}