Seems to me that since the year 0 does not exist in the timing, you must add an additional year on to the total at the end of your calculation. That would bring us to 1914.
You lost a year, you didn't gain one.
As I said,
-3 + 5 = 2 (including origin)
-607 + 2520 = 1913 (including zero year)
Subtract the zero year/origin, and it's one less not more.