Im not totally sure. I found a calendar on the forums to use for the game but I can't really pinpoint the year this all starts.
As others have stated already, there's no hard and fast date, you can adjust as necessary (the only hard date I know of is Second Darkness). However, it has to be at least 4707 if you're using any of the dates in the book. On page 27, when talking about Tsuto, it mentions 4705 and then two years of time in between. 4708 sounds like the most reasonable date to me, but really, you can run things however you want.