Neither the Christian (Gregorian) calendar nor the Jewish calendar is correct.
The Gregorian calendar was based upon an attempt to include Yeshua’s birth year in the counting of time. However, without having all the historical facts at the time that this was done, there was a discrepancy. Today, it is possible to pinpoint with a fair amount of accuracy the actual year Messiah was born by correlating Luke’s account with Matthew’s and other historical sources from this period (especially Josephus). Four basic clues will be considered.
The first clue concerns the year Herod died, which was the year 4 B.C. The Gospel accounts clearly state that when Yeshua was born, Herod the Great was alive. This means the Messiah was born before the year 4 B.C.
The second clue pertains to the decree that was issued in the days of Quirinius, or more precisely, around the year 8 or 7 B.C. Luke’s point is that the Messiah was born after Quirinius ordered the census. The first two clues combined give a three- to four-year parameter, indicating that Yeshua was born somewhere between 8 and 4 B.C.
The date can be narrowed down even further. According to Josephus, Herod left Jerusalem for Jericho in the year 5 B.C., which is where he spent the last year of his life. He died in Jericho, never again to return to Jerusalem. Matthew stated that when the wise men met with Herod, he was still in Jerusalem (Mt. 2:7, 16). We can therefore deduce from Josephus’ writings and the Gospel account that they would have arrived in Jerusalem in or before the year 5 B.C. This serves as the third clue.
The last clue concerns Yeshua’s age. When the wise men met King Herod in or before the year 5 B.C., the Messiah was already about two years old (Mt. 2:16). Putting all these clues together, we can conclude that Yeshua was born sometime between the years of 7 and 6 B.C. Hence, the Christian calendar is off by approximately seven years. For example, the year 2000 had already occurred between 1993 and 1994.
While the Christian calendar revolves around the year Yeshua was born, the Jewish calendar is tied to the year Adam was created. The present Jewish calendar is based upon a calendar put together by rabbis in the second century A.D., and they, too, did not have all of the history and details necessary to be accurate. The result is that the Jewish calendar is off by approximately 250 years.
Hence, both calendars are not 100% accurate, but it is too late to do anything about this now. However, it is recognized by scholars and historians that both calendars are inaccurate.
