The year of birth of Jesus has been estimated in a manner that is independent of the Nativity accounts, by using information in the Gospel of John to work backwards from the statement in Luke 3:23 that Jesus was "about 30 years of age" at the start of his ministry.
By combining information from John 2:13 and John 2:20 with the writings of Josephus, it has been estimated that around 27-29 AD/CE, Jesus was "about thirty years of age".
However, the common Gregorian calendar method for numbering years, in which the current year is 2011, is based on the decision of a monk Dionysius in the six century, to count the years from a point of reference (namely, Jesus’ birth) which he placed sometime between 2 BC/BCE and 1 AD/CE.
Although Christian feasts related to the Nativity have had specific dates (e.g. December 25 for Christmas) there is no historical evidence for the exact day or month of the birth of Jesus.
There have been different approaches to estimating the date of the start of the ministry of Jesus. One approach, based on combining information from the Gospel of Luke with historical data about Emperor Tiberius yields a date around 28-29 AD/CE, while a second independent approach based on statements in the Gospel of John along with historical information from Josephus about the Temple in Jerusalem leads to a date around 27-29 AD/CE.
