trait OptionalEndDateDateTimeRangeTrait in Optional End Date 8

trait OptionalEndDateDateTimeRangeTrait {

  /**
   * {@inheritdoc}
   *
   * @see Drupal\datetime_range\DateTimeRangeTrait
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    $separator = $this
      ->getSetting('separator');
    foreach ($items as $delta => $item) {
      if (!empty($item->start_date)) {

        /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
        $start_date = $item->start_date;

        /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
        $end_date = $item->end_date;
        if ($end_date !== NULL && $start_date
          ->getTimestamp() !== $end_date
          ->getTimestamp()) {
          $elements[$delta] = [
            'start_date' => $this
              ->buildDateWithIsoAttribute($start_date),
            'separator' => [
              '#plain_text' => ' ' . $separator . ' ',
            ],
            'end_date' => $this
              ->buildDateWithIsoAttribute($end_date),
          ];
        }
        else {
          $elements[$delta] = $this
            ->buildDateWithIsoAttribute($start_date);
          if (!empty($item->_attributes)) {
            $elements[$delta]['#attributes'] += $item->_attributes;

            // Unset field item attributes since they have been included in the
            // formatter output and should not be rendered in the field
            // template.
            unset($item->_attributes);
          }
        }
      }
    }
    return $elements;
  }

}