/**
 * @ORMColumn(type="date")
 */
private $start_date;

$builder->add('start_date', DateType::class, [
                'widget' => 'single_text',

                // prevents rendering it as type="date", to avoid HTML5 date pickers
                'html5' => false,

                // adds a class that can be selected in JavaScript
                'attr' => ['class' => 'js-datepicker'],
            ])

$builder->get('start_date')
->addModelTransformer(new CallbackTransformer(
    function ( $dateAsString ) {
        // transform the entity back to an integer
        return new DateTime('2019-01-01');
    },
    function ( $dateAsDate ) {
        // transform the integer to an entity object
        return '2019-01-01';
    }
));