DateTimezone example

    // DrupalDateTime object at this point. We need to convert it back to the     // storage timezone and format.
    $datetime_type = $this->getFieldSetting('datetime_type');
    if ($datetime_type === DateTimeItem::DATETIME_TYPE_DATE) {
      $storage_format = DateTimeItemInterface::DATE_STORAGE_FORMAT;
    }
    else {
      $storage_format = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
    }

    $storage_timezone = new \DateTimezone(DateTimeItemInterface::STORAGE_TIMEZONE);

    foreach ($values as &$item) {
      if (!empty($item['value']) && $item['value'] instanceof DrupalDateTime) {
        /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
        $date = $item['value'];

        // Adjust the date for storage.         $item['value'] = $date->setTimezone($storage_timezone)->format($storage_format);
      }
    }
    return $values;
  }