How could Harold Camping make another prediction of doomsday within only a couple days?

Now he's making the date October 21st, when the rapture, and the world will instantly blow up at one time. How can he do this?! How can man claim to know when God will return when it clearly states in the Bible that no man, not even Jesus, or the angels in Heaven know when Judgment day will come by doing his mathematics? The Bible also says the earth will be inhabited time indefinite, meaning forever. God made the Earth for a purpose, to make the world to where life can be enjoyable. Can man really know when the world will end?