Passing 'CET' to core_date::normalise_timezone (lib/classes/date.php) will validate and return the same value, but CET in not a valid timezone in moodle.
This appens beacause "CET" is stored in self::$bczones, so normalise_timezone return it. The string is also stored in self::$badzones corresponding to Europe/Berlin and this is the correct behaviour we expect.
