Because of a quirk in our calendar, which began on January 1 at
year 1 (1AD) we have some question about when a specific named
decade begins and ends. Since there was no year 0, between year
1 BC and year 1 AD, it can be confusing.
A decade, by definition
is ten years, regardless when you begin counting, but decade
names begin in the “1” year, not the “0” year. The 2020s will
begin in 2020, but the next named decade will begin in 2021.
When the year is 2021, 2020 years would have passed since the
beginning of our calendar at year one.
2020 is also a leap year, so we get to enjoy an additional day
of fun next year.
Incidentally,
Chronométrophilia, the Swiss Association for the History of
Timekeeping, pointed out a practical shortcut applies to the
century system. "The 20th [Century] started with 1901, ended
with 2000, so the two first digits of the last year of any
century give it its name."
Year ending in 1 is
named decade and year ending in 0 is century.