"Different legal systems adopt different rules about how far back claims can reach."

The key point is that history itself does not contain a rule saying "go back exactly X years." A legal system has to decide where to draw the line.

For example, imagine a piece of land with this history:

500 CE: Community A builds a temple.

900 CE: Community B replaces it with another building.

1400 CE: Community C builds a mosque.

1800 CE: Another structure is added.

Today: The site is still in use.

History can investigate each stage and ask:

What stood there?

When was it built?

Who built it?

What evidence supports that?

But history cannot answer:

"Which historical moment should determine today's legal ownership?"

That is not a historical question. It is a legal and political question.

If a legislature passes a law recognizing claims only from a certain historical period, then the law has selected the baseline. Another country could choose a different baseline.