That’s an interesting question. On the one hand, it feels weird that you can’t represent those dates at all.
On the other hand, representability of a given date becomes progressively less useful the further back in time you go, and stuff becomes really gnarly once you go past the Julian calendar in 45BC.
Also, simplifying to “no dates before Jan 1 0001” has very little impact on applications dealing with the modern-ish world (with “modern” generously defined as “anything after the collapse of the Roman Empire”), and I can only assume applications dealing with earlier times could do with a more specialised representation for dates anyway.
That seems to be...a problem? How do you deal with archeological dates, of which there are many, in Ion?