Symfony2: poor man's date_range form type

DateRangeType.php

<?php

namespace Ormigo\Bundle\OrmigoBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

use Symfony\Component\OptionsResolver\Options;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

use Ormigo\Bundle\OrmigoBundle\Form\Validator\DateRangeValidator;

class DateRangeType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('start_date', 'date', array_merge_recursive([
                'property_path' => 'start',
                'widget' => 'single_text',
                'format' => 'yyyy-MM-dd',
                'model_timezone' => 'UTC',
                'view_timezone' => 'UTC',
                'attr' => array(
                    'data-type' => 'start',
                ),
            ], $options['entry_options']))
            ->add('end_date', 'date', array_merge_recursive([
                'property_path' => 'end',
                'widget' => 'single_text',
                'format' => 'yyyy-MM-dd',
                'model_timezone' => 'UTC',
                'view_timezone' => 'UTC',
                'attr' => array(
                    'data-type' => 'end',
                ),
            ], $options['entry_options']))
        ;

        $builder->addEventSubscriber($options['validator']);
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => '\DatePeriod',
            'entry_options' => [],
            'validator' => null,
        ));

        $resolver->setAllowedTypes([
            'validator' => 'Symfony\Component\EventDispatcher\EventSubscriberInterface',
        ]);

        // Those normalizers lazily create the required objects, if none given.
        $resolver->setNormalizers([
            'validator' => function (Options $options, $value) {
                if (!$value) {
                    $value = new DateRangeValidator(new OptionsResolver());
                }

                return $value;
            },
        ]);
    }

    public function getName()
    {
        return 'date_range';
    }
}

DateRangeValidator.php

<?php

namespace Ormigo\Bundle\OrmigoBundle\Form\Validator;

use DateTime;

use Symfony\Component\Form\FormError;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class DateRangeValidator implements EventSubscriberInterface
{
    public function onPostBind(FormEvent $event)
    {
        $form = $event->getForm();

        $datePeriod = $form->getNormData();

        if ($datePeriod->getStartDate() > $datePeriod->getEndDate()) {
            $form->addError(new FormError('date_range.invalid.end_before_start'));
        }
    }

    public static function getSubscribedEvents()
    {
        return [
            FormEvents::POST_BIND => 'onPostBind',
        ];
    }
}