How to use timestampable entities with api-platform / symfony serializer

DoctrineExtensionTimestampableInterface.php

<?php

namespace AppBundle\Entity\Doctrine;

interface DoctrineExtensionTimestampableInterface
{
    /**
     * @return \DateTime
     */
    public function getCreatedAt();

    /**
     * @return \DateTime
     */
    public function getUpdatedAt();
}

services.yml

services:

    app.serializer.normalizer.timestampable:
        public: false
        class: AppBundle\Serializer\Normalizer\TimestampableNormalizer
        arguments:
            - '@api_platform.serializer.normalizer.item'
        tags:
            - { name: serializer.normalizer }

TImestampableNormalizer.php

<?php

namespace AppBundle\Serializer\Normalizer;


use AppBundle\Entity\Doctrine\DoctrineExtensionTimestampableInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

class TimestampableNormalizer implements NormalizerInterface
{
    /**
     * @var NormalizerInterface
     */
    private $normalizer;

    public function __construct(NormalizerInterface $normalizer)
    {
        $this->normalizer = $normalizer;
    }

    /**
     * @param DoctrineExtensionTimestampableInterface $object
     * @param string                                  $format
     * @param array                                   $context
     * @return array
     */
    public function normalize($object, $format = null, array $context = array())
    {
        $normalized = $this->normalizer->normalize($object);
        $normalized['createdAt'] = $object->getCreatedAt();
        $normalized['updatedAt'] = $object->getUpdatedAt();

        return $normalized;
    }

    public function supportsNormalization($data, $format = null)
    {
        return $data instanceof DoctrineExtensionTimestampableInterface;
    }
}