2009 years since when?

Answer 1

Had the Roman abbot Dionysus Exiguus been correct, it would now be 2009 years since the death of King Herod the Great. He knew that it was impossible to say when Jesus was born, but he knew, or thought he knew, when Herod died. So, he devised the new Christian calendar to begin on the year of Herod's death.

He based the date of Herod's death on the reign of the Roman emperor Augustus, but was unaware that Augustus only adopted that name four years after his reign began, going by his birth name of Octavius until then. So, Exiguus commenced his calendar just 4 years too late.