Why is the theory that Jesus would return in 2000 considered false? Couldn't it still be true?

I'm not hard core religious or anything, but I always heard that Jesus was supposed to return in 2000. That still might be the case, which would make the neo-jesus somewhere around 11 years old by now. Why do people dismiss that theory so fast? I'm just curious, perhaps I don't have all of the information, but I'd assume that if he came back, it would be in the 2020s or 2030s.

It always seemed to be a simple thing to me.