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When I define a decade, it contains 10 years, starting from '1 and ending on '0.

Specifically, I mean by '1-'9 being 1st-9th and '0 being 10th.

Since the current decade, namely the 2010s, ranging from 2011 to 2020, is about to end.

One time me and some other users were having a conversation on MSE about how the decades are being ordered.

I understand what User45266 is saying, since saying "the 30s" and including 40 (?!) is not very logical. But (strictly speaking) it is '1-'2-'3-'4-'5-'6-'7-'8-'9-'0, not '0-'1-'2-'3-'4-'5-'6-'7-'8-'9. For '0 to be the 1st year of the decade there would have to be a "year 0" (?) and there isn't in the common sense.

Did the 2010s already end? Or is it still in progress (but about to end)? Does the 2020s contain the years 2021-2030 or 2020-2029?

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    The word "decade" can be used refer to a ten-year period with any starting point: e.g. in 2027, it will be possible to say "It has been a decade since 2017". By analogy with the usage with centuries, the time period from the start of 2011 to the start of 2021 could be referred to as the "202nd decade". – herisson May 5 at 7:00
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    Since AD1 (or 1CE) is the first year, every decade and every century should end with a zero. But in common parlance the forties, e.g., begins in 1940 and ends with 1949. The turn of the century was celebrated as the calendar turned to 2000. That’s the culture. – Xanne May 5 at 7:39
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    Technically, this is a sort of "fencepost problem" involving "counting from zero". A discussion of it Is found here: forum.gcaptain.com/t/…. Because of its hackish nature, it might have great success (or possibly a previous answer) on one of the number or computacional Stacks. – Conrado May 5 at 8:10
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    You started the question by saying, "when I define a decade." If that's how you've defined it, then that's what it means to you. – Jason Bassford May 5 at 15:54
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    Does this answer your question? Does the nth century contain the (100n)th year? – Jason Bassford May 5 at 15:57
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A decade is any 10 year period regardless of when it starts. The time from your birthday until your next birthday is one year even if you weren't born at new years. (unless of course you were born on a leap day) Similarly 2010-04-05 08:31 UTC to 2020-04-05 08:31 UTC is a decade. (although you do run into issues with different calendars having different ideas of what a "year" is, )

We do however have names for particular decades the same way we give names to particular years, even though the choice of the new year is just as arbitrary.

"The 2020s" covers 2020 to 2029 inclusive. This naming scheme for decades identifies them by the digits in the decimal representation of the year numbers.

However, when we say something like "the third decade of the twenty-first century" we run into the complication that we start counting at 1 CE instead of having a year 0. So the 3rd decade of the twenty-first century is actually 2021 to 2030 inclusive and 2020 when I am writing this is in fact the last year of the second decade of the twenty-first century not the first year of the third.

These two different ways of naming decades are offset by a year from one another. Neither one of them is more correct than the other though. They are perfectly legitimate decades being named either way.

The 2010s are over, but the 202nd decade CE still has about 8 months left (as of this answer)

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