The mathematical and philosophical question: the 20s began with self-isolation, Gulfy and Alyonka, or did they end with the 10th?
There are people who believe that the decade ends in the year "for nine." That is, the 80s began in the 1980s, and ended in 1989, the 90s passed from the 90th to the 99th-well, and so on. This is mathematically not very correctly, because there was no zero year in history, after the minus of the first, the first one immediately began, and the first decade ended not in the ninth year, but in the 10th, but you can understand them purely intuitively.