well then the new millennium should been on the 1/1/2001 rather than 1/1/2000(base on your take). Because we celebrate the new millennium on 1/1/2000 then the decade after that is 1/1/2010.

You're right, and so it was, at least to all that are able to count. You see, it was on 1st of January 2001 that 2000 years have passed since we started counting AD. Well, more or less - truth be told, our current calendar was adjusted many times after and I don't mean just leap years. As far as our calendar is concerned, we changed the year page 2000 times on 1st of January 2001 and the Earth rotated around the Sun close to 2000 times (about 20 degrees more, but that's an approximation out of my head, I'll finish with my morning coffee 1st before engaging ) on that same date. If you really like round numbers that much then feel free to celebrate the new millennium any time you feel so, tho don't be too offended if I won't ask for your help in any math related problems

But you missed another point. For all intention and purposes each year divisible by 10 is the definition of start of a decade.

It is the same reason as why year 2000 is a millennium, not 2001.

Right, just like you were 1 year old on the day you popped out of your mom's womb? You were not, you were 0 years and few seconds old. For all intentions and purposes? You mean astrology here?

No, for practicality and convention.

I didn't say you were wrong, but you missed the main stream view point. I agree that 2010 isn't really the start of a new decade if you trace back to AD 1, however, mainstream view and convention is that each year divisible by 10 is the defined to be start of a decade, no reason other than convention, same as how Millennium is defined.

Gotcha there, and you're also correct in that. There's another point tho why I personally don't find 2010 as round of a number as 2009, because it just isn't. 2011 is gonna be even less "round" tho To put it into "perspective" i think it was either Jimmy or Alan Carr in one of the new year's panels shows (Alan Carr in Chatty Man or Jimmy Carr in Big Fat Quiz, I honestly don't remember there were so many of them) that made a funny remark on it using 2009 and then 2010 shaped party glasses... Can't see much through 2010 shaped ones LOL... was funny