That’s the reason why the rapture didn’t occur as predicted on Saturday:
A California preacher who foretold of the world's end only to see the appointed day pass with no extraordinarily cataclysmic event has revised his apocalyptic prophecy, saying he was off by five months and the Earth actually will be obliterated on Oct. 21.
Harold Camping, who predicted that 200 million Christians would be taken to heaven Saturday before catastrophe struck the planet, apologized Monday evening for not having the dates "worked out as accurately as I could have."
I’m not a theologian, but I suspect the reason why Christ said that no man knows the day or hour of his return is because no man knows the day or hour of his return. Not even Harold Camping.
As such, the problem with his prediction isn’t the math; it’s the fact that he’s trying to predict the unpredictable. Naturally, his future revisions will be as doomed to failure as his original one.