> an extra 5 weeks will have passed for the probe.

What does this mean? The only quantity I can see with an obvious meaning is that the probe will arrive somewhere after experiencing a certain amount of time since leaving Earth. But the time of the probe's arrival is not easily matched to any time on Earth.

Suppose the probe arrives and dispatches a message which travels back to Earth at the speed of light. For simplicity, the distance is exactly 4 light years.

- How much time did the probe experience between leaving Earth and sending the "I made it" message?

- How much time did Earth experience between the probe leaving and the "I made it" message arriving?

It's kind of unsatisfying that these two questions don't share both their endpoints, but at least they both have well-defined answers.