Who am I to argue with these guys? They obviously got the whole return of Jesus figured out. Gawker has the breakdown:
The billboards are produced by supporters of a Christian network called Family Radio Inc., and they’ve set up a website to spread the Good Word, WeCanKnow.com.
According to Camping’s prediction, the Rapture will happen exactly 7,000 years from the date that God first warned people about the flood. He said the flood happened in 4990 B.C., on what would have been May 21 in the modern calendar. God gave Noah one week of warning.
Since one day equals 1,000 years for God, that means there was a 7,000-year interval between the flood and rapture.
STREET KNOWLEDGE 
The date has passed, but many more dates will arise and some already have. Of course, the scripture declares this knowledge is and will be unknown. – http://www.rogertharpe.wordpress.com