In the book of Matthew we have an important bit of information:
Mat 2:1 Now when Jesus was born in Bethlehem of Judaea in the days of Herod the king, behold, there came wise men from the east to Jerusalem,
The Jewish historian Flavius Josephus tells us that Herod died shortly after an eclipse of the moon (Antiquities of the Jews, Book XVII, Chapter VI, end of 4th paragraph), but prior to Passover (Wars of the Jews, Book II, Chapter I, paragraph 3.). This is an extremely important bit of information, because astronomers today know that there were eclipses of the moon on the following dates:
A total eclipse on March 23rd, 5 BC at 8.52 pm
A partial eclipse on March 13th, 4 BC at 4.04 am.
A total eclipse on January 10th, 1 BC at 1.35 am.
Historians have determined, on this and other evidence, that King Herod apparently died in the Spring of what we now call 4 B.C. (Julian year 4710), probably early in the Jewish month of Nisan (April), after the partial eclipse of the moon occurring on March 13th, 4 B.C. If you will recall, Herod lived long enough after Jesus' birth to order the slaughter of the infants in Bethlehem, in an attempt to murder Jesus-
Mat 2:16 Then Herod, when he saw that he was mocked of the wise men, was exceeding wroth, and sent forth, and slew all the children that were in Bethlehem, and in all the coasts thereof, from two years old and under, according to the time which he had diligently inquired of the wise men.
This would suggest that Jesus was born in either 5 or 6 B.C., as Herod set an upper age limit of 2 years for the infants to be slain. So, on this evidence alone, the current method of numbering the years since the birth of Jesus is in error by at least 4 years, and perhaps as much as 5 years.
http://www.hol.com/~mikesch/year.htm