I think a better way is to make an option into admincp and make that field required or not. If is not required the date will be displayed. Let me know what you think.
You code is related to this, in XenResource/ControllerPublic/Resource.php:
if ($extraData['version_string'] === '')
$extraData['version_string'] = XenForo_Locale::date(XenForo_Application::$time, 'Y-m-d');