Perhaps the most depressing thing about Camping's misadventures is to realize just how easy it was for him to recover from a false prediction that was easily demonstrated.
He predicted the end in 1994. When it didn't happen, he simply shrugged his shoulders and said 'I got it wrong, but I'll get it right next time.' And people believed him!! In the interview from the first post in this topic, he is smugly mocking the prediction of 2012! As if he had not already gotten one prediction utterly wrong!
Here is how he came up with the 2011 date (from Scientific American):
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.
This is the guy who was mocking everyone else for getting end-of-world dates wrong!!
(Edit to add a link to the above quote: https://blogs.scientificamerican.com/observations/judgment-day-math-the-numbers-behind-harold-campings-may-21-claim/)