Десятилетие начнётся 2021-м.
С наступающим!
There are two methods of counting decades in recognition. One, called ordinal, counts decades starting with the first year 1 CE (For example, the years 1981–1990 is referred to as the 199th decade), while the other, called cardinal, groups years based on having the same digits (For example, the years 1980–1989 is referred to as the 1980s, or commonly known as the eighties).
https://en.wikipedia.org/wiki/Decade Но поскольку ссылаться на Wiki - моветон, то вот ссылка на более солидный источник:
https://www.timeanddate.com/calendar/decade.html It all comes down to how we talk about time spans. With few exceptions, we usually think about centuries and millennia as numbered entities, counted up from year AD 1, such as “the 21st century” or “the third millennium.”
Decades, however, are commonly categorized based on the year numbers. For example, we say “the eighties” instead of “the 199th decade.” Similarly, the upcoming decade is technically the 203rd decade, but we call it “the twenties.” According to this common definition, decades generally encompass the time span from years ending with 0 to years ending with 9, such as 2020 – 2029.
А посему не будем спорить, а просто поздравим друг друга еще раз с новым годом!
Он у меня уже почти взрослый - пошла 98-я минута!
