When does the first decade of the 21st century end?

As we near this year's end, we are being bombarded with articles that summarize the past decade..."The best baseball (football, soccer, badminton) players of the decade!"..."The top news stories of the decade!"...etc. But, when does the decade end? December 31, 2009 or 2010?

I am reminded of a question posed to my class by a law school professor in 1970. "When will the century end," he asked. There were two obvious answers: December 31, 1999 or 2000. As we pondered the answers, it became obvious the question had nothing to do with law. It was asked to make us think.

Ultimately, we figured out that in the calendar in common worldwide usage, the first year was "Year 1" and the 100th year was "Year 100." That marked the first century. Each succeeding century, then, began with "1" and end with "00." The twentieth century began on January 1, 1901 and ended on December 31, 2000. The first decade of the twenty-first century began January 1, 2001 and will end about a year from now, on December 31, 2010. Save those "all-decade" lists for another year.

A present-day law school professor ponders this same question here. Maybe this is about law?