They probably went back and looked at ancient records, in addition to simple math.
I am taking that into consideration. Ancient records (where you can find them) are not all that accurate. I have read several accounts that conjecture that Halley's Commet may have been the Star of Bethlehem.
Good Grief, I don't think they even have George Washington's birthday accurate. It's actually a different day from what we celebrate. And that was less than 300 years ago