RFC 2550 Section 3.1 has years from 0000 to 9999 as four digit but zero padded (so the fall of Rome was 0476). It then gets appropriately weird as it was published April 1, 1999.

You might also enjoy the Kurzgesagt human era calendar - https://youtu.be/29pN-2KM2DI - https://shop-us.kurzgesagt.org/collections/calendar