I think it is almost for certain that they will come up with a new date within the next 20 years or so.
I believe there is something in the psyche of apocalyptic millennialists that CRAVES a date (but not a fig <-- a pun! get it?).
The ideal date has to be within the imaginative reach of the target audience, so that most of them can visualizr themselves making the goal, yet it should also be far enough away that each peon is highly motivated to assure his salvation by working extra, EXTRA hard and making personal sacrifices that any normal person would consider either stupid or insane.
Yes, they will find a new date, and then they will deny it after it passes.