Quote Originally Posted by DennyW
Actually, I rechecked, and my software did say 10, not -10.
Do the math. If the intake doesn't open until after TDC and the exhaust closes before TDC, there is negative overlap.

Overlap, definition: number of crankshaft degrees during which both intake and exhaust valves are open.

If the exhaust closes before the intake opens, then there can be no overlap.