DateToStringTransformer.php

<?php
class DateToStringTransformer implements DataTransformerInterface
{
    /**
     * @param mixed $value
     * @return mixed|void
     */
    public function transform($value)
    {
        if (!$value) {
            return null;
        }

        return $value->format('Y-m-d');
    }

    /**
     * @param mixed $value
     * @return mixed|void
     */
    public function reverseTransform($value)
    {
        return \DateTime::createFromFormat('Y-m-d', $value);
    }
}

SomeType.php

<?php
class SomeType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) use ($builder) {
                //$game = $event->getData();
                $form = $event->getForm();

                $period = new \DatePeriod(
                    \DateTime::createFromFormat('Y-m-d', '2014-01-01'), // for test
                    new \DateInterval('P1D'),
                    \DateTime::createFromFormat('Y-m-d', '2014-01-30') // for test
                );

                $dates = array();

                foreach ($period as $date) {
                    $dates[$date->format("Y-m-d")] = $date->format("j/M");
                }

                $form->add($builder->create('date', 'choice', array(
                        'label' => 'form.game.date',
                        'empty_value' => 'form.game.date',
                        'auto_initialize' => false,
                        'required' => false,
                        'choices' => $dates
                    ))
                    ->addModelTransformer(new DateToStringTransformer())->getForm()
                );
            });
    }

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

SomeTypeTest.php

<?php
class SomeTypeTest extends TypeTestCase
{
    /**
     * @test
     */
    public function testSubmitValidData()
    {
        $begin = new \DateTime();
        $formData = array(
            'date' => '2014-01-15'
        );

        $type = new SomeType();
        $form = $this->factory->create($type);

        $form->submit($formData);

        $this->assertTrue($form->isSynchronized());
        $this->assertEquals(['date' => \DateTime::createFromFormat('Y-m-d', '2014-01-15')], $form->getData());

        $view = $form->createView();
        $children = $view->children;

        foreach (array_keys($formData) as $key) {
            $this->assertArrayHasKey($key, $children);
        }
    }
}