The birthdate for the world is found in the writings of the Roman astrologer Julius Firmicus Maternus, who wrote his text on astrology sometime between 334 and 337 AD. He claimed that he got the dates from two older astrologers, Asclepius and Anubius, none of whose writings survive -- nor does Firmicus give their reasons for the date.

As for Bishop Ussher's date, hmm. I thought he'd given 9:00 am as the time!