Simple Form with datefield - Test case for https://github.com/silverstripe/silverstripe-framework/issues/8056

PageController.php

<?php

namespace {

    use SilverStripe\CMS\Controllers\ContentController;
    use SilverStripe\Forms\DateField;
    use SilverStripe\Forms\FieldList;
    use SilverStripe\Forms\Form;
    use SilverStripe\Forms\FormAction;

    class PageController extends ContentController
    {
        /**
         * An array of actions that can be accessed via a request. Each array element should be an action name, and the
         * permissions or conditions required to allow the user to access it.
         *
         * <code>
         * [
         *     'action', // anyone can access this action
         *     'action' => true, // same as above
         *     'action' => 'ADMIN', // you must have ADMIN permissions to access this action
         *     'action' => '->checkAction' // you can only access this action if $this->checkAction() returns true
         * ];
         * </code>
         *
         * @var array
         */
        private static $allowed_actions = ['Form'];

        protected function init()
        {
            parent::init();
            // You can include any CSS or JS required by your project here.
            // See: https://docs.silverstripe.org/en/developer_guides/templates/requirements/
        }


        public function Form()
        {
            $dateField = DateField::create('birthdate', 'Your Birthdate')
                ->setMaxDate('-21 years')
                ->setHTML5(false)
//                ->setDateFormat('MM/dd/yyyy')
                ->setValue('2018-05-08');

            return new Form(
                $this,
                'Form',
                new FieldList([$dateField]),
                new FieldList([FormAction::create('test', 'Test')])
            );
        }

        public function test($data, $form)
        {
            var_dump($data);
            die('test');
        }
    }
}