If it says 'for a limited time', then wouldn't tying itmto the lifetime of the author be unconstitutional? Because strictly speaking it's impossible to predict whether an author's life will end.

Decided in Eldred v. Ashcroft. As long as Congress stipulates a non-infinite copyright term, it's constitutional. Given that the chance of a human being eventually ceasing to live has been 100% so far, it's legitimate to assume that life + n years is still a finite period of time.

It's in fact trivially easy to predict whether an author's life will end.

Watch: I predict an author's life will end. I give it very high odds indeed.