class DateRangeCustomFormatter

class DateRangeCustomFormatter extends DateTimeCustomFormatter {
  use DateTimeRangeTrait;
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'separator' => '-',
    ] + 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] = [
            'start_date' => $this->buildDate($start_date),
            'separator' => [
              '#plain_text' => ' ' . $separator . ' ',
            ],
            'end_date' => $this->buildDate($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['separator'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Date separator'),
      '#description' => $this->t('The string to separate the start and end dates'),
      '#default_value' => $this->getSetting('separator'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    if ($separator = $this->getSetting('separator')) {
      $summary[] = $this->t('Separator: %separator', [
        '%separator' => $separator,
      ]);
    }
    return $summary;
  }

}