The important things have already been said:
1) numerology is crap.
2) calendars are man-made
Here is a list of calendars:
https://secure.wikimedia.org/wikipedia/en/wiki/List_of_calendars
The Gregorian calendar is just one out of many. So why cherry-pick this specific calendar, and why cherry-pick two arbitrary years from it? Of course, this is done in order to manufacture a scary-sounding "prediction". Which, as already said, does not even add up. LOL!
And the reason people choose these specific two dates are probably
a) the Japan earthquake is recent news (but whoever came up with this "calculation" messed the date it happened)
b) the 9/11 is the date most Americans immediately connect with emotionally, plus it is the (or one of the) most significant historical events in the lives of many young American people born in the late 1980's or early 1990's. "There was no history or the world before I was born" kind of thing.
Yeah, this "prediction" is pretty much confined inside United States AFAIK. The inclusion of 9/11 date is a tell-tale sign.
Why not pick other arbitrary dates like:
April 4, 1905: the Kangra earthquake in India. 20,000 people killed.
Or:
January 23, 1556: Shaanxi earthquake in China. About 830,000 people killed.
Or any other earthquake from this LONG list:
https://secure.wikimedia.org/wikipedia/en/wiki/List_of_earthquakes
Not to mention the millions of other natural and man-made disasters that have happened throughout history.
You can pretty much take Dec 21st, 2012 and shuffle numbers in your calculator to get a couple of suitable dates, mash them together and claim it's a "prediction". It's just totally random.