We don't base our year system around a fictional character. That is proof that Jesus was (and still is) real. You can argue that He wasn't divine in any way if you like but He most definitely existed.
I know there is this recent atheistic movement to change "B.C." and "A.D." to "B.C.E." and "C.E." but no matter how much they want to change or erase history, the birth, life, death, and resurrection of Jesus Christ was the true turning point in history.
They are the events that divide the "old" from the "new". They shook history up so much that we arranged our year system around them. If those events were indeed fictional as you believe then we would not have done that.