Does the first year of a decade count as part of it?

@SW-User 2010-2019 [b]is[/b] ten years...isn’t it ? And what would be “political” about it ?

@bijouxbroussard My bad, I think just miused the term, lol. When I was a girl, I remember people debating that the new millennium, and thus the 2000's would actually start on January 1st, 2001 instead of 2000. Likewise, I remember having a phone debate with a college roommate in late 2009, that the 2010's would actually start on 1/1/2011, instead of 1/1/2010. That couldn't be though, because that's only 9 years, is what I was referring to.

@SW-User I can understand the analogy that there is no Year 0. But for decades...I was born Jan 1, 1960, and am considered to have been born in the 60s, not the 50s. If you count the years themselves:
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
They do add up to 10 years.

@bijouxbroussard Make the same backwards till you will find yourself counting the "first" decade as years:

0000
0001
0002
0003
0004
0005
0006
0007
0008
0009

And so, for doint it, includding the inexistent 0 year.
If you take it as ordinals (first year, second year…)

0001 (as 1st of the decade)
0002
0003
0004
0005
0006
0007
0008
0009
0010 (as the last one includded in same 1st decade)

@CharlieZ I don’t accept it in that regard. While the year 0 doesn’t exist, the years 10, 20, 30, 40, 50, 60 [b]do[/b]. The 60s start at [b]60[/b].

@bijouxbroussard Well, I could say that if we agree that there was not a 0 year, to count 10 years for the first decade, the year 10 had to be the 10th, so part of the first one.
But, of course, it´s only technical, a thing of mathematicians and historians.
I also use to define the 60´s as you do, dear friend.
Together with the 70´s, great decades!
(and I had hair at the top of my head by those years) 😁