public function dateTimeISO8601Local($subject) : bool
    {
        // Ugly method name because \DateTime uses strict acronym camelCasing.

        if ($subject === null) {
            return false;
        }
        $v = '' . $subject;
        return strlen($v) <= 19 && !!preg_match('/^\d{4}\-\d{2}\-\d{2} \d{2}:\d{2}(:\d{2})?$/', $v);
    }