Considering the numerous exemptions including the one for "national security," I wonder how long it extends the time limit on actual government evidence of extraterrestrial UFO's.
I'd imagine that if there were any UFOs and they were classified it would probably be similar to the "born secret" clause that nuclear weapons research gets. So if you discovered anything it would retroactively be classified even if you didn't know it at the time and you could be prosecuted (and worse) for sharing it.
/removes tinfoil hat