http://crd.ndl.go.jp/reference/modules/d3ndlcrdentry/index.php?page=ref_view&id=1000137319
When Dionysius Exiguus computed the date of Christ's birth in the Middle Ages, he named the year of the Nativity 1 A.D., and stated that Jesus' birthdate was December 25 of that year. The year immediately before this was the year 1 B.C. Whether from mathematical ignorance or design, he did not include a year zero.
This complicates the calculation of the dates of the Jubilee. Christmas of the year 2 A.D. was the 1st anniversary of Christ's birth, according to Dionysius' calculations; similarly, the second anniversary of that birth fell in the year 3 A.D. Taking this forward a few centuries, we find that the 2000th anniversary of Christ's birth should fall on December 25, 2001.
To complicate matters further, it seems that Dionysius' made an error in his calculations. Herod the Great, who the Bible says was alive at the time of Christ's birth, died in the year 4 B.C., based on the reports of Josephus. According to the Gospel of Matthew, when Herod was unable to trick the astrologers into leading him to the Child, he ordered the slaughter of all the male babies in Bethlehem. Since Herod's command (which is not attested outside the Gospels, but is consistent with his historical character) was to kill all babies under age 2, this event occurred no more than 2 years after Christ's birth. If we assume that this happened near the end of Herod's life (which seems likely), this puts Christ's birth in the year 5 or 6 B.C.
In that case, the 2000th anniversary of Christ's birth has already gone by, having been in 1993 or 1994. Naturally at the distance of years, it is practically impossibile to say with certainty what year Christ was actually born, though sometime between 7 B.C. and 1 B.C. seems all but certain.
http://www.catholiceducation.org/en/controversy/common-misconceptions/in-what-year-was-christ-actually-born.html