[dinde]

The people who believe in these types of predictions don't put much stock in science anyway. Someone who doesn't believe in carbon dating isn't going to listen to an argument regarding the Earth's age based on carbon dating.

If you really want to provide a counter in these types of discussions it is more helpful to speak in their language. My favorite example of this was when last year astronomers announced that the zodiac signs were incorrect due to a shift in the planet's position relative to the sun since the time that the zodiac was created. This produced more reverberations in the astrological community than any number of studies showing the falsehood of their predictions.

In this case, it may be helpful to point out that the December 21 is simply the date at which the next cycle in the Mayan calendar begins, as occurs every 400 years or so, and that the Mayans themselves made no predictions about cataclysmic events occurring on these dates.