You're right about one part of your post: The Rapture is not in Revelation.
It is mentioned in numerous other references, with the best, IMO, being 1 Thessalonians 4:16-17. The words in 17 "caught up", is where the word Rapture comes from the Latin for "caught up"
Other references: Matthew 24:40-41, John 14:1-3, Acts 1:9-11, 1 Corinthians 15:51-52, Philippians 3:20-21.
If it happens tomorrow as the old geezer says, it happens. If it doesn't, it sure will eventually, on God's timetable.
The whole "caught up, or the Latin rapture idea" did not come from Darby. It is a central theme of Christianity, that Darby expounded upon, and the term "rapture" came from the translation of the New testament from the Latin, and does not conflict with the Greek and Hebrew manuscripts.
Not all Christian groups, or even churches within a denomination, believe in the Rapture, though. I have a hard time seeing their viewpoints, because it directly conflicts with Biblical teaching.
But, to each their own.
I don't know if the leader of this "May 21" theory has ever read Matthew 24:36 -
"But of that day and hour, no man, no, not the angels of heaven, but my Father, only."
I wouldn't hold my breath waiting for it.
If the date comes and goes there will be a lot of relieved Jews and alot of fundamentalists rethinking their support of Israel