class TimestampItemNormalizer

class TimestampItemNormalizer extends FieldItemNormalizer {
    
    /**
     * {@inheritdoc}
     */
    protected $supportedInterfaceOrClass = TimestampItem::class;
    
    /**
     * {@inheritdoc}
     */
    protected function normalizedFieldValues(FieldItemInterface $field_item, $format, array $context) {
        return parent::normalizedFieldValues($field_item, $format, $context) + [
            // 'format' is not a property on Timestamp objects. This is present to
            // assist consumers of this data.
'format' => \DateTime::RFC3339,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    protected function constructValue($data, $context) {
        if (!empty($data['format'])) {
            $context['datetime_allowed_formats'] = [
                $data['format'],
            ];
        }
        return [
            'value' => $this->serializer
                ->denormalize($data['value'], Timestamp::class, NULL, $context),
        ];
    }

}