Gravity gets double-negated when you reverse time. Its units are over seconds squared instead of just seconds.
A simple way to see this is to imagine taking a movie of a heavy back arcing upward through the air then back down. What do you see when you run the movie backwards? A ball arcing upward them downward. The acceleration still points in the same direction: down.
A simple way to see this is to imagine taking a movie of a heavy back arcing upward through the air then back down. What do you see when you run the movie backwards? A ball arcing upward them downward. The acceleration still points in the same direction: down.