I say that 100% of them do not know that it was all cribbed from the Adventists and 95% do not know that the actual prediction was for "the end" in 1914.
When I was a child, my Catholic aunt made me read some anti-Witness pamphlet written by her church. It talked about various end-of-the-world predictions Witnesses had made over the years. I don't reacall the exact nature of what was said, but I remember believing it was an obvious lie made up to make the Witnesses look bad. It never occurred to me that the Witnesses might have actually made such predictions. In fact, I'd been repeatedly told by the congregation that such things were lies.
After all these years, I wish I could somehow get my hands on that pamphlet again. I have a feeling everything in there was actually true.