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?
Why though? In 1969 a ball appeared in the middle of the room, hit a wall and came back towards the portal…
Anything coming “back” (which kind of back?) through the portal was not in your scenario specification…
what if the portal didn’t exist in 1969? The ball travels to 1969 and is unable to come back.