Now, yes, that being said, some prophecies proclaimed in the Old Testament did occur within the time range given, for instance, the Flood of Noah's day. But for the most part that is not the case.
If you look at the Old Testament prophecies, they were to occur "in the latter days," or "a time afar off," etc. No actual dates were given. I'm of the opinion that many of the warnings given by today's "truly biblical" teachers will indeed occur, just not in the time span we expect to see.
Someone once said that we will have a better understanding of end times prophesy AFTER it occurs, which means we'll arrive at that understanding when we are on the other side of Eternity, not this side. So until then, as the scriptures say, "study to show thyself approved," and be prepared as that day will happen on a day we least expect it.
The long and short of it is that we need to be ready whatever happens because there are no guarantees in this life except that it will end.