Maybe you've seen the ads—on a billboard, on the subway, on the side of an RV. Maybe you've encountered the believers in person. However you found out, there's a good chance you've heard claims about something big happening Saturday, May 21.

That date, according to Christian radio broadcaster Harold Camping and his followers, is Judgment Day. This is no mere prediction, according to the Web site of Camping's Family Radio Worldwide: "The Bible guarantees it!" First will come a massive earthquake, powerful enough to throw open all graves. Then will follow a slow dying off of all nonbelievers until the end of the world in October.

An "infallible, absolute proof" of Camping's assertion rests on a head-spinning numerological argument about the number of days that have elapsed since Jesus was crucified. The date of the crucifixion is itself somewhat uncertain, but Camping takes it to be April 1 in 33 AD. Come May 21, 2011, Camping says, 722,500 days will have elapsed since that occurrence. And 722,500 is (5 x 10 x 17) x (5 x 10 x 17). Those numbers are important, according to Camping, because 5 symbolizes atonement, 10 represents completeness, and 17 is for heaven.

Why does 5 symbolize atonement? Here we turn to Exodus 30:15: "The rich shall not give more, and the poor shall not give less than half a shekel, when they give an offering unto the Lord, to make an atonement for your souls." For those of you who prefer fractions, "half a shekel" would be 1/2 shekel; the decimally minded might favor 0.5 shekel. Camping is evidently in the latter camp; he takes this verse as evidence of 5's association with atonement.

The figures and math get wispier from there. To quote from Family Radio's "infallible proof": "The number 10 or 100 (10 multiplied by 10) or 1,000 (100 multiplied by 10) signifies completeness.... The Bible speaks of [Satan] being bound 1,000 years to signify that he was bound for the completeness of God's plan, which in actuality of time was 1,955 years." As for 17? "God instructed Jeremiah that this purchase of [a field] for 17 shekels was done as a guarantee or demonstration that the time would come that Israel would again occupy Jerusalem. That is, people again would go to Heaven."

Let's assume for the moment that the numerological treatment has some merit, that 5, 10 and 17 are somehow as biblically important as Camping believes. A purely mathematical complaint is that (5 x 10 x 17) x (5 x 10 x 17) is a completely arbitrary way to factor 722,500. A more concrete way to break down a large number into smaller numbers is to factor it into primes, which in the case of 722,500 yields the unique solution 2^2 x 5^4 x 17^2.

In other words, one could make a more compelling numerological argument for the importance of 722,500 if one started with a foundation of 2, 5 and 17 rather than 5, 10 and 17. (One could easily ignore the exponent 4, which is just 2 + 2, 2 x 2 or 2^2.) As noted above, atonement could just as easily be 2 or 5 (depending on whether one looks at "half" as 1/2 or 0.5), so why not both?

That's the problem with numerology, of course—you can find meaning in just about anything, and just about any way you want, if you look hard enough. And the May 21 prediction is certainly not the first time that prognosticators have taken certain patterns (if one is so generous as to call them that) to be omens of import. As Scientific American columnist Michael Shermer noted in a Wall Street Journal op-ed, human history is threaded by predictions of the apocalypse. We at Scientific American have even speculated on several (nonbiblical) ways that the world as we know it might come to an end.

It should be noted that Camping himself has made such predictions—and garnered news headlines—before. Back in the 1990s he said he was "more than 99 percent sure" that the end was coming in September 1994. That date came and went, but this time Camping is absolutely certain. Given his fluidity with numbers, maybe he will be 110 percent sure the next time around.