The debate rages on. Does the new decade begin in 2020 or 2021?
One camp says the 20s begin in 2020.
The other camp says there was no year 0. Therefore, the decade begins with 1 and ends with 0.
Actually, both are correct. A decade is defined as a 10-year period. 2005-2014 is a decade.
It is all semantics. This article really explains it nicely.