I am studying valve timing diagrams. Many of them show something like (for instance) the exhaust valve stays opens till ~20 degrees after TDC and the intake valve opens ~12 degrees before TDC. Wouldn't this mean that the valves are open and pushed into the combustion chamber during TDC?

If so I have two questions,

1. Wouldn't the piston hit the valve and damage it as it reaches TDC?
2. The explosion wouldn't be so good since there would be ports still open when the spark fires, making poor compression?

What am I missing? Am I reading the timing diagrams wrongly?

EDIT: I would like to add tags like camshaft, timing diagram, valve timing, piston, TDC etc but they are not in the predefined list.