he New Decade actually starts on January 1, 2021.”””
NO- Mathematically, that date is the end of the FIRST year of the decade. 1-2-3-4-5-6-.....
2020 thru 2029 is the decade.
2000 thru 2009 was the first decade of this century.
Because there was no Year “A.D. 0”, modern date keeping started on January 1, A.D. 1. The first new decade therefore started on January 1, A.D. 11, and so on.
Thus the current new decade starts on January 1, A.D.2021