class UserSerializationSubscriber implements EventSubscriberInterface
{
    private $coreTwigExtension;

    private $user;

    public function setCoreTwigExtension(TwigExtension $coreTwigExtension)
    {
        $this->coreTwigExtension = $coreTwigExtension;
    }

    public function setUserService(UserService $user)
    {
        $this->user = $user;
    }

    public static function getSubscribedEvents()
    {
        return array(
            array(
                'event' => 'serializer.pre_serialize',
                'method' => 'onObjPreSerialize',
                'class' => 'SomeBundleEntityEntityClass',
                'format' => 'json'
            )
        );
    }

    public function onObjPreSerialize(PreSerializeEvent $event)
    {
        $context = $event->getContext();
        $context->attributes->get('groups')->map(
            function(array $groups) use ($event) {
                if (in_array('somegroup', $groups)) {
                    $obj= $event->getObject();

                    if ($obj->getConfirmedOn()) {
                         $contextualDate = $this->coreTwigExtension->getContextualDate($obj->getConfirmedOn());
                         $event->getVisitor()->addData('displayConfirmedOn', $contextualDate);
                    }
                }
            }
        );
    }

some_bundle.handler.serialization:
    class: SomeBundleHandlerObjectSerializationSubscriber
    calls:
        - [setCoreTwigExtension, ['@bundle_core.twig.extension']]
        - [setUserService, ['@some_bundle.service.user']]
    tags:
        - { name: jms_serializer.event_subscriber }