class DateTimeDefaultFormatter

class DateTimeDefaultFormatter extends DateTimeFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'format_type' => 'medium',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  protected function formatDate($date) {
    $format_type = $this->getSetting('format_type');
    $timezone = $this->getSetting('timezone_override') ?: $date->getTimezone()
      ->getName();
    return $this->dateFormatter
      ->format($date->getTimestamp(), $format_type, '', $timezone != '' ? $timezone : NULL);
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $time = new DrupalDateTime();
    $format_types = $this->dateFormatStorage
      ->loadMultiple();
    $options = [];
    foreach ($format_types as $type => $type_info) {
      $format = $this->dateFormatter
        ->format($time->getTimestamp(), $type);
      $options[$type] = $type_info->label() . ' (' . $format . ')';
    }
    $form['format_type'] = [
      '#type' => 'select',
      '#title' => $this->t('Date format'),
      '#description' => $this->t("Choose a format for displaying the date. Be sure to set a format appropriate for the field, i.e. omitting time for a field that only has a date."),
      '#options' => $options,
      '#default_value' => $this->getSetting('format_type'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    $date = new DrupalDateTime();
    $summary[] = $this->t('Format: @display', [
      '@display' => $this->formatDate($date),
    ]);
    return $summary;
  }

}