2010 is the final year of the decade, not the first. The reason is that our calendar began at the year 1. that is, we went from 1 BC to 1 AD. There was no year zero. Is the twelfth egg the last egg of a dozen or the first egg of the next dozen? We begin counting from one, not zero. By the same reasoning, the 21st century began January 1, 2001, not 2000.