The decade ends after the "10" year. Everyone who celebrated the new millennium in the year 2000 is stupid. Think of it this way:
If you started the calendar today, it would be the year 1. If a decade has 10 years, then year 10 would still be in the same decade as year one.
The current decade, century and millennium started in 2001.
If you started a new calendar it would be year 0 until 365 days later when it would be year 1. This is much the same as when you have a baby it is not 1 when it pops out... It is 0 (and 1 day or whatever), and it is only 1 after 365 days. Thus, 2000 was the beginning of a new decade/century/millennium...
You cannot honestly tell me that 5 billion people were wrong on this matter...
Also, by that logic, the "60's" lasted from 61-70, the "70's" from 71-80, and so on. A decade starts on the 0 and ends on the 9 by common standards
What they did when they started the CE calendar is started with year 1. Making the first day the "baby being born" Year 1, day 1. So the first ten years are 1-10. The first 100 years 1-100. The first 2000 years 1-2000. Thus the start of the 21st century being 2001.
I am not saying either of you are right, but just acknowledging a large group of people think the same way as you doesn't make it right.