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),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function hasCacheableSupportsMethod() : bool {
    return TRUE;
  }

}