No one recognized year 1 at the time, it was assigned year 1 status retroactively.
A decade by definition is ten years, any ten consecutive years. We can start and end them whenever we wish. 2005-2014 was a decade.
The 1920's (1920-1929) was the decade of the 1920's. 1921-1930 was also a decade in length, but it would not be accurate to call that the "twenties" as that leaves out 1920 but includes 1930.
The 20s was 21-30. Its not our fault that people attribute things to the wrong decades. While a decade is just 10 years (like saying im a decade old at age 10) when you are talking decades in reference to the established date system it is year one to year ten so 21-30 is the decade of the 20s. It doesnt matter how silly it sounds to say 1930 is in the 20s... because it IS. 1930 was part of the 20s and 1990 was part of the 80s.
This year (2020) is the last year of the decade of 2010s.
