Why can this be accessed directly? Is there a specific reason why it can be accessed directly?
https://github.com/cakephp/cakephp/blob/3.0/Cake/Utility/Time.php#L33
> The format should use the locale strings as defined in the PHP docs under strftime` (http://php.net/manual/en/function.strftime.php)

With a setter/getter we could make sure the format is valid... or at least thow an exception of the format to be set is invalid...