The late Stephen J. Gould and the late Arthur C. Clarke got into a good-hearted dispute about whether the new millennium started January 1, 2000, or January 1, 2001. Gould even wrote a wonderful, tidbit-filled book called Questioning the Millennium: A Rationalist's Guide to a Precisely Arbitrary Countdown.
"Precisely arbitrary" captured it exactly right. But we can be counted upon to argue about when each decade and each century starts right up until the fourth millennium begins on January 1,
3000 3001 3000.
If it really matters to you - and if it does, you might try getting out more - you can dig as deeply as you want into this dispute. For instance, check out what Jan Zuidhoek has to say at Millennium Mistake. And there's also this.
At its root, the argument stems from the fact that the creators of the Western calendar were not Mayans or Hindus, peoples with both the concept and a symbol for zero. Hence, our calendar recognizes no year zero. Every decade begins not in the year ending in a 0, but ending in a 1, 2011, not 2010.
The only problem being, that in popular parlance, it doesn't make sense to call the decade of the '90s, 1991-2000. And how does 2011 fit into the decade of the '00s?
Advocates of the no-year-zero approach argue that the 6th century priest-scholar Dionysius Exiguus forces us to accept that the new decade won't start until 2011. It was he who first calculated in AD 532 (by means not wholly clear) the time when Jesus Christ was conceived and born. Exiguus apparently knew about the concept of zero, but he didn't have the symbol and wrote his conclusions with Roman numerals. He went directly from 1 BC to AD 1, dates now scientifically notated as 1 BCE and 1 CE.
Subsequently, when it was discovered that the time in office of Quirinius (the Roman governor of Syria) and the death date of Herod (the Roman puppet-king of Judaea) didn't match up with biblical references for them, the 1 BCE calculation flew out the window. Using these references, the actual third millennium may have begun in 1996.
Whether or not you believe Jesus even existed, however, let's cut poor old Dionysius Exiguus some slack for argument's sake. Assume that he was right in his probably politically motivated calculation that Jesus was born December 25, 1 BCE. Exiguus, who apparently understood zero but had no symbol for it, did what historians still do - set the beginning of the year 1 CE right after 1 BCE. No zero in between. So, that put the beginning of Jesus's second decade on the planet in the year 11 CE and the second decade of the third millennium at 2011.
But if there were a year zero, which, logically speaking, there must be, it would mean that the second decade of Jesus's life began in the year 10 CE, and thus 2010 marks the beginning of the second decade of the third millennium.
When astronomers count, they do include a year zero, thanks to the work of Jacques Cassini in 1740. Many countries have legally adopted this approach to counting the decades and centuries. For astronomers, the new decade begins in 2010.
So, 12 months from now, when you encounter somebody arguing whether the second decade of the 21st Century is a year old or just beginning, make a suggestion that they take on a topic more worthy of exploration: such as whether Spam is real food.