FDA (flashback data archive) question

So it seemed like we had a perfect case for using FDA. We have a table with a large amount of rows many of them are no longer needed except for historical purposes. So I setup a testcase. For some reason I was thinking that you could control what date columns for the FDA to use when issuing the "as of" query, but I haven't seen how to do that in any of the docs. Am I off track here? We are using 18c on Oracle Linux. Test case is pretty simple, I made