Symfony Serializer Serialize DateTime instance

app config services.yml

services:
    datetime_normalizer:                                                                                                                                
        class: Acme\Foo\Normalizer\DateTimeNormalizer
        tags:
            - { name: serializer.normalizer }

src Acme Foo Normalizer DateTimeNormalizer.php

<?php

namespace Acme\Foo\Normalizer;

use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

/**
 * Class DateTimeNormalizer
 * @author Julien Deniau <julien.deniau@mapado.com>
 */
class DateTimeNormalizer implements NormalizerInterface
{
    /**
     * {@inheritdoc}
     */
    public function normalize($object, $format = null, array $context = array())
    {
        return $object->format(\DateTime::ISO8601);
    }

    /**
     * {@inheritdoc}
     */
    public function supportsNormalization($data, $format = null)
    {
        return $data instanceof \DateTime;
    }
}