The calendar is intended to mark the number of years since the
death of King Herod the Great.
The Roman abbot Dionysus Exiguus devised the new Christian
calendar in 533. He knew that it was impossible to say when Jesus
was born, but he knew, or thought he knew, when Herod died. So, he
chose to begin his Christian calendar on the year of Herod's death,
and he based this on the reign of the Roman emperor Augustus.
Unaware that Augustus only adopted that name four years after his
reign began, going by his birth name of Octavius until then,
Exiguus commenced his calendar just 4 years too late.