It's the Millennium all over again. (Which began 1/1/01)
You are not "1" at birth.
A set of ten ends with (wait for it) ten. It does not begin with zero. There is no zero in date keeping unless is appears as a 10, 100 or 1000. It never appears as "0."
The decade ends in one more year. The new decade begins on 1/1/21. 2020 is the "ten" in this story. It completes the final year of the prior set of ten.