None of the GB had any religious training as far as I know, and they don't really care to. So they can make any claim and go about their day because it just doesn't matter. They are focused on $$ and keeping their jobs and golden living arrangements.
I was still in when a program came on PBS about apocalyptic religions through the centuries. I'm sure your list above was part of the program. The show came right through the 1800's and the offshoots of Miller. Each sect presented perfectly good reasons in their mind why their specific date was "the real end of the world." It made all the difference to me and I was out within two years. I watched the same show with an elder friend and his wife, and it didn't phase him one bit. So it goes.