This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
There are two reasons for this:
- There is no year 0.
- Jesus was born before 4 BC.
The concept of a year "zero" is a modern myth (but a very popular one).
Roman numerals do not have a figure designating zero, and treating zero
as a number on an equal footing with other numbers was not common in
the 6th century when our present year reckoning was established by
Dionysius Exiguus (see section 2.13). Dionysius let the year AD 1
start one week after what he believed to be Jesus' birthday.
Therefore, AD 1 follows immediately after 1 BC with no intervening
year zero. So a person who was born in 10 BC and died in AD 10,
would have died at the age of 19, not 20.
Furthermore, Dionysius' calculations were wrong. The Gospel of
Matthew tells us that Jesus was born under the reign of king Herod the
Great, who died in 4 BC. It is likely that Jesus was actually born
around 7 BC. The date of his birth is unknown; it may or may not be 25