Matthias\ProjectBundle\DependencyInjection\Configuration::getConfigTreeBuilder PHP Method

    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('matthias_project');
        $rootNode->addDefaultsIfNotSet()->children()->scalarNode('start_at')->isRequired()->validate()->ifTrue(function ($value) {
            return strtotime($value) === false;
        })->thenInvalid('"start_at" should be parseable by strtotime()')->end()->end()->scalarNode('deadline')->isRequired()->validate()->ifTrue(function ($value) {
            return strtotime($value) === false;
        })->thenInvalid('"deadline" should be parseable by strtotime()')->end()->end()->end();
        return $treeBuilder;
    }