When the engine is timed with a timing light, at idle, with the vacuum advance blocked, I think it is called 'initial advance'. If you add this value (in degrees) to whatever value you achieve with centrifugal and vacuum advance, you get 'total advance.
I don't think 'initial advance' will ever be ATDC, or retarded. Every timing value I have ever seen is BTDC.
So, technically, timing will never be retarded. It might be 'less advanced' but never ATDC.
I think this is correct!
Stuart
