public static function validateTimeFormat($format)
    {
        $format = str_replace(array_keys(static::$_map['time']), '', $format); // remove placeholders
        $format = preg_replace("/'(.*)'/", '', $format); // remove literals
        $format = preg_replace('/[^a-z]/i', '', $format);
        $format = trim($format);

        return empty($format);
    }