I think Camping is saying the Rapture on May 21st will last 153 days..(Jesus and the catching of the 153 fish in John :21) and end October 21st, 2011 which will signal the Great Tribulation based on the Feast of Tabernacles 1260 days which end on Passover April 4th 2012 Passover...
That is what I think he has done with those dates...
Camping is teaching that Judgement Day is May 21st. On that day the graves will be opened and the bodies of the elect that have already died will be transformed into glorified bodies and will be raptured along with all the remaining elect who are still alive.
The bodies of the unsaved dead will be scattered all over the earth and those who remain alive will stay on the earth where there will be suffering for 153 days until the whole world is destroyed by fire on October 21st.