Other way around. You can move the start date all the way back to 1, but you can't move up the end date beyond 731.
For example, let's say you wanted to make a mod that covered the years 1 AD to 730 AD, you could change the calendar from AVC to AD and have it start in 1 AD. I don't know of any calendar that starts around the time Rome starts, but if there is one, you could use it to extend the base game (would take a lot of revising to the history files though).
The point is, the game can potentially go from year 1 to 730. It can't start earlier than 1, it can't end later than 731.