class DateTimePlainFormatter

class DateTimePlainFormatter extends DateTimeFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            if (!empty($item->date)) {
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
                $date = $item->date;
                $elements[$delta] = $this->buildDate($date);
            }
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function formatDate($date) {
        $format = $this->getFieldSetting('datetime_type') == DateTimeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
        $timezone = $this->getSetting('timezone_override') ?: $date->getTimezone()
            ->getName();
        return $this->dateFormatter
            ->format($date->getTimestamp(), 'custom', $format, $timezone != '' ? $timezone : NULL);
    }

}