Look at something where it is truly impossible to predict a date. Like the collapse of the money system--I can predict a tentative date with the full knowledge that they are capable of kicking that can a few years down the road. Like the total collapse of the USA as a free country--again, I can predict sometime after Perry becomes the president with the knowledge that he could make a wise decision (or one that will delay the inevitable) or that it will happen even sooner. These things, you can predict the events but with the knowledge that those in charge might delay things.
As for predicting a definitive date, you are at the mercy of the inevitable "kicking the can down the road" by the leaders. And predicting divine retribution of any kind is always dangerous--especially since Jehovah only wants to dangle this crap in front of mankind so we will have something to look forward to in return for obeying harsh rules. The damage caused by obeying the rules is real--wasted opportunities and ruined lives. The reward, that has been predicted ever since sometime before Jesus. So not only can the date be way off, but even the actual events are probably going to go bust as well.
We saw that on May 22, 2011. We saw that on June 13, 2007. We just saw September 30, 2011--and I even gave that the benefit of the doubt by allowing it to mean the last place on earth to see September 30. And, every time someone predicts that God is coming on such and such a date, he does a no show. At least when I predict disaster, I am predicting it based on the inevitable end results (when you put money into the system, people will lose faith in it and hyperinflation is inevitable; when you regulate industry to death, jobs become scarce and unemployment is sure to go way up and companies will abuse their employees that much more). I also never predict absolute dates. I sometimes try to estimate, but with the knowledge that the leaders could stall and do one (or more) evasive measures that prolong the inevitable. But I will never give a "believe or die" date that uses complicated formulas.
And by the way, December 22, 2012 will come to the western-most parts of the earth (I know of no country that is in that time zone), and December 23, 2012 will arrive in Wellington, and nothing will happen out of the ordinary. That might be the time frame when global Agenda 21+ happens, or a major war, or your currency becoming toilet paper--or just 2 more shopping days before Christmas. And I am not willing to stake my life, or anyone else's, on anything important happening then.