Krish, you have to count whole years. If you mean 2000 - 2010, you need to count the year 2000 as well as 2010. Why? Because you are non-specific. It may be that you count Jan 1, 2000 - Dec. 31, 2010, or Dec. 31, 2000 - Jan 1, 2001. Common practice is to count a decade 2001 - 2010, which is 10 years by definition. Thus 2000 - 2010 is not a decade but 11 years. (qed). Reason is the nature of 0. 0 has a special status in group theory because it's not really defined. It is just defined as the neutral element and if you add or substract it to a number, the number does not change. There is therefore no year 0, either, but there is 2 BC, 1 BC and then follows 1 AD, 2 AD for the same reasons stated above.
Sorry...