We already agree that our calendar is wrong, off by anywhere from 2 to 16 years, depending on who's doing the reckoning, and further in error because of the absence of a year zero.
The more fundamental point is that God did not intend us to mark His years by the birth of Jesus.
If He had intended this we would have a Biblical fixing of the date.
Further, the day of Jesus' birth is unremarkable as all men are born.
However, very few return from the dead, that event is remarkable, and it is the defining moment of Christianity, the very moment of proof that his sacrifice was not in vain. And the Bible gives a precise reference for when this happened!
Clearly this was the date the calender was supposed to start!
For extra points, this makes our calender off by ~ 33 years. That makes this Holy Year 1972, giving us about 28 years to get our affairs in order before the real end of the millennium...