Interesting to see the issues you’ve wrestled with. Although Movable Type works well with very old dates, it did require some slight hacking on my part to make things work well when newer dates were expected.
For example, I want to give each entry of the diary the correct 17th century date, but I also want the system to publish the entries automatically on the correct 20th century day. So I had to add something to the MT code for that specific blog to add 343 years to each entry when working out what to automatically publish.
I’ve been meaning to write up what I’ve done on that front for years; I’ll try and do so this week.