There is a pedantic argument that because there wasn't a year zero, each new decade/century/millennium ends with the year ending zero and begins with the year ending one. HOWEVER, while technically correct in a sense, it's based on the fallacy that these groupings of 10/100/1000 years are an innate mathematical principle, rather than being a retrospective grouping that we arbitrarily apply to them. We group them based on patterns, and the pattern that they share are the first one, two or three digits respectively. Therefore it's absolutely fine to group a decade starting with the one ending zero, it just means that the first such grouping only has nine years in it (or ten, if you allow it to include the year 1 BC). Furthermore, if anyone pedantically insists that the decade doesn't start until 2021, then you're entitled to point out back to them that they must call it "the two hundred and third decade" and not "the 2020s".