Dear internet people who flunked basic math. Counting starts at 1. The first decade of the Gregorian calendar started with year 1. The second decade started with year 11. The 203rd decade of the Gregorian calendar starts in year 2021.
When asked about the dispute, Rick Fienberg of the American Astronomical Society says he doesn't think his group has adopted an official position on the matter — but he adds, "History is clear: Because there was no Year Zero, the first decade of the common era (CE or AD) was years 1 to 10, the second decade was years 11 to 20, and the next decade will be years 2021 to 2030."