You are in an empty room, standing near a wall facing the opposite wall. You roll a ball and measure the time it takes the ball to hit the opposite and reach back to you.

You repeat this again - but now in the middle of the way there is a time traveling portal, and in the other side is the same middle of the room but in 1969. The ball maintained its speed vector in the transition.

How long does it take the ball to reach you?