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?
It depends.
Assuming it comes back through the portal, and it took 4 seconds with no portal.
Your stopwatch will measure 4 seconds when the portal is there.
A stopwatch in the ball will measure 4 seconds when the portal is there.
If you want to take stepwise measurements based on your clock, the ball will be at time 1 sec before going through the portal, time negative 1,767,225,598 when it hits the wall, 3 sec after it exits the portal second time, 4 sec when it reaches you.
