But the decade is from 2011 to 2020 so we still don't know.
Seems like this is a myth. I read about this elsewhere, and i'm glad we can forget about this. While this backwards logic is used in the Ordinal system, this is not the standard way of counting decades. The Cardinal system is, and this puts decades in groups with the same digits. Thereby dismissing the dumbass that decided Jesus was born at the start of the second year (after a full "year 1"). You can also check that "the 1900's" are indeed the decade from the start of 1900 till the end of 1909.This.
I'm glad we can all forget about this ridiculous notion and sleep well at night, with the knowledge that the balance in the universe has been restored.