class DateRangeCustomFormatter

class DateRangeCustomFormatter extends DateTimeCustomFormatter {
  use DateTimeRangeTrait;
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return static::dateTimeRangeDefaultSettings() + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    // @todo Evaluate removing this method in
    // https://www.drupal.org/node/2793143 to determine if the behavior and
    // markup in the base class implementation can be used instead.
    $elements = [];
    $separator = $this->getSetting('separator');
    foreach ($items as $delta => $item) {
      if (!empty($item->start_date) && !empty($item->end_date)) {
        /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
        $start_date = $item->start_date;
        /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
        $end_date = $item->end_date;
        if ($start_date->getTimestamp() !== $end_date->getTimestamp()) {
          $elements[$delta] = $this->renderStartEnd($start_date, $separator, $end_date);
        }
        else {
          $elements[$delta] = $this->buildDate($start_date);
        }
      }
    }
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form = $this->dateTimeRangeSettingsForm($form);
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    return array_merge(parent::settingsSummary(), $this->dateTimeRangeSettingsSummary());
  }

}