"Since the year zero does not exist", but the number 0 does. It is a legitimate number on the line, as much so as any integer.
So without the year zero/origin, you have to subtract one, not add one.
If you have to add one, please explain how.
Yes, you lost a year so the 2520 years goes farther.
Okay! Now I get it. I'll accept that, and admit that I have been conflating the length of 2520 years with the timeline itself.
Now I understand.