Technically wasn’t year zero relative to the birth and death of Christ? If you are Christian that is..
It's hard to put into words, technically time is measured before and after Christ. Christ taking human form is important but to us, His resurrection is a more important event.
IE us Assyrians call Resurection Sunday (AKA Easter) as the great celebration (Eida Goorah) and Jesus' birth (AKA Christmas) as the small celebration (Eida Soorah).
So If I ran things I'd make the Resurrection as year zero.
But Jesus has always been there on the right hand of the Father, since Genesis 1:1. Now before becoming Man, Jesus was referred to as the "Word".
John 1:1 (KJV) In the beginning was the Word, and the Word was with God, and the Word was God.
Jesus became Christ when He was born Man.
Sorry if I waffled on, it's difficult to put into human words to be understood by human minds.