Millenarianism is a belief that society is approaching a transformative period, and many wack job Christians reach for the Book of Revelations to bear that out. The doomsters are the same thing, it's just that most of the them aren't taking it very seriously. It's a kind of hobby, unless they are seriously prepping. Putting some kind of magical timetable on human events is something that communists, fascists, theocrats, and other evil doers like to cook up as well.