How did Harold Camping figure out there's supposed to be an earthquake in New Zealand?

He says that and i already thought he was crazy but he has a time AND date..i will accept that he believes his numbers make sense but where does the Bible say "and on the 21st day of the fifth month of the year 2011 God proclaimed There shall be a huuuuge earthquake in New Zealand at 6 pm" and how could he possibly gather that mathematically?