このような感じでフォームを作成した場合など、ちゃんと「date_default_timezone_set('Asia/Tokyo');」など設定してないとエラーがでます。

$form = $app['form.factory']->createBuilder('form', $data)
->add('run_time', 'date', array(
'widget' => 'single_text',
'input' => 'string',
->getForm();
)

Exception: DateTime::__construct(): It is not safe to rely on the system's timezone settings. 
You are *required* to use the date.timezone setting or the date_default_timezone_set() function.
In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier.
We selected 'Asia/Tokyo' for 'JST/9.0/no DST' instead