Or it's actually the end of the last ten-year period, because when you count to 10 starting with 1*, you don't get there until you get to 10. And then the next 10 starts with 11.
Don't be mad. Or do. Let it out.
Math and I are here for you.
* I'm told there is no year 0 (no Chinese year of the goose egg**).
Of course, time is relative and anyone can call anything whatever they like, but according to the system that we're using, there's no number 0, so we started with 1 and finish with 10, unless there was a 9-year decade somewhere in the past two-thousand years.
Or, if some amount of time was added to or subtracted from each year or decade as necessary to get us to the point we're at now, such that 2010 can be the first year of a new decade.
Which I'm fine with, either way, if that's how people want to go.
I'm agreeable. Especially when the issue doesn't really matter.