2000 is popularly thought of as the first year of the new millennium but since 1 is the first year of the Common Era, that makes 2001 the first year. With that in mind, is 2000 the last year of the 1990s or does it belong to the current decade?
2000 is, by definition, not part of the 1990s. It is part of the last decade of the second millennium A.D., though.
(I’m not Christian, but I don’t hold with that Common Era stuff; it isn’t as if the year 0 on the Roman calendar was a significant date in history for any reason beyond the (incorrectly calculated) birth of Jesus.)
I know it is a strange question and while I tend to aree that it is 1990-1999, I’m hesitant because that would mean the first decade of the Common Era wasn’t, in fact, a decade.
Some here are confusing the Gregorian calendar, which was introduced in 1582 (and which does not deal with counting years), with the Anno Domini year counting system, which is a thousand years older. In any case, there was no year “0” in the Anno Domini system. The year 1 B.C. was followed directly by the year 1 A.D.
Any 10 years in succession is a decade. Centuries are any 100 years in succession. Even if it was 1743 - 1842. We refer to them any way we like, but when we use a term like “The Nineties” it’s pretty specific about which decade we are referring to, and why.
We do have “Ages” of course. I like to think we’re currently in “The Communication Age” because of the predominance of things like cellphones, the internet, and interactive television. But those don’t have specific beginnings and endings down to dates.
When dealing with pure digit specifications, though, you have to stick with the mathematics.
The key words here are “cardinals” vs “ordinals”. When you refer to the 1900s, the 90s, etc. you are using cardinals: actual values of numbers. So the 1900s went from 1900 to 1999, and the 90s from 1990 to 1999 (or 1890 to 1899) etc.
But if you refer to ordering, then you are talking about ordinals. So the first century lasted form 1 to 100, etc. up to the 20th century from 1901 to 2000. Note the usage of “first”, “second”, nth in general. So the last (another “ordering word”) decade of the 20th century was from 1991 to 2000.
Note that this “one off” of numbering for cardinals vs. ordinals is common: during your second year of life you are one year old.
All this could have been avoided if Computer Scientists had been allowed to set things up: we like to start our counting from zero.
The logic for including 2000 as the last year of the 20th century does not apply to naming decades. If you were to talk about the ninth decade of the 20th century, you would technically be referring to 1991-2000 because of the year 0 thing mentioned earlier. Saying “the '90s” is a different thing altogether, and only to the most misguided technical pinheads among us means anything other than 1990 - 1999.