Since the world is ending today, Google figured we might as well go with a bang. Granted, we may go with a bang whether we like it or not if a huge (invisible) asteroid smashes into our planet, for example.
In any case, Google decided to mark the occasion, perhaps for future alien archeologists, with a doodle depicting the end of the Mayan calendar. The doodle is live on Google UK.
Well, the calendar doesn't exactly end, it just hit the last page. That's like worrying about Armageddon each time your wall calendar hits December 31.
Specifically, December 21, 2012 marks the end of the 13th Baktun or B'ak'tun, making 12/21/12 22.214.171.124.0 in the Mayan calendar.
A Baktun is equal to 20 Katuns, while a Katun is in turn equal to 20 Tuns and so on. The Mayan calendar is a lot more complex and much, much longer than anything we have now.
For example, as a unit of time, the 'millennium' is the biggest thing we've got. The Baktun is approximately 394 years, for comparison.
But beyond the Baktun, Mayans had the Piktun, which is equal to 20 Backtuns or 7,885 years; the Kalabtun – 157,704 years; the K'inchiltun – 3,154,071 years; and the Alautun – 63,081,429 years. So mayans could simply say that it's been about one Alautun since the dinosaurs went extinct.