While it's true that it's a certain kind of insanity to believe the date of the end of the world can be discerned through Biblical prophecy, even insanity needs a reason to settle on one date rather than another, and given that the Bible focuses on extremely ancient events I was curious what such a reason could be.
It turns out there are three different lines of investigation favoring May 1, 2011. This is interesting, because if all these lines were independent, it would provide some circumstantial evidence that something was up: either that the date was correct, or at least that the Bible authors really intended this date. So let's see what's going on:
The three lines of evidence I could find, not all of which are espoused by the same people, are:
1. 7000 years after the Flood
2. 722,500 days after the Crucifixion of Jesus
3. 42 years after the rebuilding of Jerusalem
I'll start by giving these the most sympathetic explanation I can, then discuss why it might not be such a good idea to sell everything you own just yet.
SEVEN THOUSAND YEARS AFTER THE FLOOD
The Bible says that on the seventeenth day of the second month, God spoke unto Noah, saying in seven days He would destroy the world. The rest is history. However, the New Testament Book of Peter says that to God, a thousand years is as a day.
So maybe there was a coded meaning in God's statement to Noah. In addition to the world-destroying Flood that was going to occur in seven literal days, there was also a more final destruction of the world that would occur in seven days God-time, ie seven thousand years.
Since Noah's Flood was in 4990 BC , and since the seventeenth day of the second month on the Hebrew calendar corresponds to May 21 on the Gregorian calendar, God will destroy the world on May 21 2011 AD - that is, 7000 years after He first made that statement.
722,500 DAYS AFTER THE CRUCIFIXION
Christ was crucified on April 1 33 AD, in what may have been the most distasteful prank in history ("April Fools, you guys! I'm really still alive!"). May 21, 2011 is 722,500 days from this date. And as Scott Adams once said, and I think rightly, "The Lord works in mysterious ways, but He uses a base ten system and likes round numbers."
But it's actually more than that. In some systems of Biblical numerology, the number five equals "atonement", the number ten equals "completeness", and the number seventeen equals "heaven". (5x10x17)^2 = 722,500. So the number 722,500 has mystical properties relating to the combination of atonement, completeness, and Heaven - exactly the sort of mystical properties one would expect to find in the number of days until the Apocalypse - and what's more, it has them twice.
42 YEARS SINCE THE REBUILDING OF JERUSALEM
The prophet Daniel declared that it would be sixty two and seven and one sevens from the re-building of Jerusalem to the coming of the anointed one, leading everyone around him to apparently nod and say "Oh, that makes perfect sense, we'll just write it down without asking you to clarify further."
Although "sevens" usually means "week" in Hebrew, if you take it to mean "seven year period" then adding the 62 and the 7 sevens to get 69*7 = 483 years, which is more like 476 years when you convert the Hebrew calendar from the Gregorian. 476 years from the time a Persian king gave the order to rebuild the walls of Jerusalem puts us right smack in the middle of the ministry of Jesus, confirming we're on the right track.
But why divide the prophecy into a 62*7 and a 7*7 period if we're just going to combine them? Perhaps because it's meant to be used twice. The second time Jerusalem was rebuilt was in 1969, after the Israelis recaptured it in the Six-Day War. This time, if we use only the 7 seven period and not the 62 seven one, we get 49 years later = 2018. And since the Apocalypse is going to last seven years , the Rapture will begin in 2011. May 21 seems as good a date as any, to take advantage of the fine spring weather.
SO WHAT'S WRONG WITH THIS PICTURE
The first thing that's wrong with this picture is that they're trying to determine a date when millions of people will float up into the air based on adding numbers in a 3000 year old anthology of legends. But the second and more interesting thing that's wrong with this picture is that they're not doing a very good job.
Let's start with the Flood. God says that in seven days there will be a Flood. Taking the Bible at face value, in seven days there was a Flood. It was a pretty big one. The statement doesn't need any further meaning. If no Flood happened, then you might think "Hmmm, what was God really trying to say?". In this case, water thousands of feet deep covering the Earth seems like a pretty good bet at why this guy kept talking about a flood coming.
In fact, God says more than this: He says the Flood will come in seven days, and last for forty days and forty nights. According to the Bible, this is what happened. If we interpret the seven days to refer to seven thousand years, we leave ourselves open to accusations of pushing the forty days and forty nights under the table (will the apocalypse last for forty thousand years? Did the writers of the Warhammer 40,000 universe know something the rest of us don't?)
Also, why date the flood to 4990 BC? This is a particular idiosyncracy of Camping's, the man pushing the whole May 21 Rapture date. The Bible has a lot of ages and begats, along the lines of "Bob lived for 520 years and begat Ethel; Ethel lived for 260 years and begat Winnifred", but with more multisyllabic Semitic names.
Camping would like to be able to add all of these numbers up to get a simple calendar. For example, we can add 520+260 = 780 and so there were 780 years between the birth of Bob and the birth of Winnifred. The problem here is that the Bible makes it pretty clear that the ages given are the final ages at death, and so Camping's method would only work if we assumed people had their first child at exactly the moment they died.
Camping doesn't believe that, but he does believe the ancient Hebrews used a calendar in which they chose as a new patriarch the person who died at the exact instant of the last patriarch's death. So if Bob was the patriarch, people would say "It's now Year 200 of the Age of Bob", and then when Bob died they'd be without a calendar so they'd discover that the most recent birth was Ethel, and proclaim "All hail the new Age of Ethel!". When it says "Bob begat Ethel," it only means that Ethel was in the direct line of Bob. Why Camping thinks the ancient Israelites would work this way is unclear; maybe to make life easier for future Biblical historians?
In any case, once you add up the ages of everyone in the Bible like this unto the time of Christ, you get 11023 BC for the Creation of the World and 4990 BC for the flood. Needless to say, everyone else gets different numbers: the Jews use 3760 BC for Creation (and flood obviously later), and Ireland's own top Biblical historian, the famous Bishop Ussher, got 4004 BC for Creation (and again, flood later).
Also, what's this "seventeenth day of the second month is May 21" crap? Okay, you're using the Hebrew calendar, I get that.
Moving on to the numerology argument. I'm not going to argue that five is atonement and ten is completion and so on. I am going to argue that every number has a numerological meaning approximately as apocalyptic-sounding as "atonement" or "completion". Let's choose a number at random - let's take a page from Douglas Adams and go with 42. According to list of significance of numbers in Judaism, 42 is the number of letters in one of God's divine names. But it's also 6 (representing creation) times seven (representing completion; apparently a lot of things represent completion). The completion of creation - that sure sounds apocalyptic to me!
Further, because the date of the Crucifixion is unclear, Camping actually had several dates to choose from when searching for numerological significance. The most commonly cited date by scholars is April 3, leading to 722,498 days, which is sort of disappointing...although still two (representing the Devil) times seven (representing completion) times 51607 (representing oil filters).
And moving on to the prophecy of Daniel: this is my favorite. Most scholars agree Daniel was written in 150 BC or so, then postdated to 520 BC to make its prophecies look amazing (it worked). The prophecies indicate a date from the destruction of Jerusalem in 586 BC, to its rebuilding by Cyrus in 538 BC, all the way to the problems with Antiochus Epiphanes (the bad guy in the legend of Hanukkah) in 160 BC, all as understood by people who had a poor grasp of history and/or simple addition.
But what's great is that even though the author of Daniel cheated by predicting things that already happened, he ended up accidentally predicting an event a hundred fifty years in his own future, as well. Because if you ignore Cyrus rebuilding Jerusalem in the 6th century BC, and concentrate on Artaxerxes rebuilding Jerusalem in the 5th century BC, and convert the lunar years into solar years, and add two of the numbers together, you get a year smack in the middle of the life of Jesus.
This is a mildly interesting coincidence, but you've got to consider the number of degrees of freedom involved. First, there's which rebuilding of Jerusalem you want to go with. Second, there's whether you want to use the 62 week period, the 69 week period, or the 70 week period. Third, there's whether you want to convert to solar years or leave as lunar years. Fourth, Jesus lived for about thirty five years, so you've got a lot of time to work with.
Let's go with Douglas Adams again and say I wanted to make the prophecy of Daniel hit the year 42 AD. It's as simple as this: start with Artaxerxes' reconstruction of Jerusalem in 445 BC. Add the sixty-nine seven period for 483 years - now we're in 39 AD. Get halfway through the last seven, and the midpoint of the Apocalypse falls squarely in 42 AD.
This should make us very suspicious of the second calculation - 49 years since the rebuilding of Jerusalem in '69 - even before we find out that a previous version of this argument said 49 years since the rebuilding of Jerusalem in '67, and that what actually happened in '69 was the Israeli parliament approving the creation of a "Company for the Reconstruction and Development of the Jewish Quarter of Jerusalem". God may work in mysterious ways, but I really hope that He's not basing the end of world on legislation governing the Israeli bureaucracy.
So overall, the evidence for a May 21 2011 beginning of the Apocalypse is little better than you probably expected. Although three arguments do settle on the same year (and to some degree the same day), they all have so many degrees of freedom that with sufficient ingenuity you can make them settle on any time you want. There's nothing in any of this that can't be explained by coincidence, sketchy numerology, and good old motivated cognition.