Answer
The special benefit of prophecy is that no one can prove a prophecy wrong until the time for its fulfilment. On the other hand, a prophecy that is too far off in the future will probably be ignored and then forgotten, unless intermediate 'signs' are also prophesied. So, the recent prophecies of the end of the world in the year 2012 fit this mould perfectly - far enough away when first proposed that they have not yet been proven wrong, but near enough to seem relevant. As 2012 approaches, with the inevitable disappointment that the world did not end after all, a new, corrected date such as 2015 becomes the focus of attention.
Mark's Gospel provides the earliest well-known example of a prophecy of the end of the world, when it has Jesus predict the end of the world and his return in clouds of glory within the lifetimes of his audience. This failed to happen and the later gospels said that no one but God knows when Jesus will return. Since then, there have been numerous prophecies of the end of the world, all fortunately incorrect.
A more successful concept of the gift of prophecy is to take an earlier book, usually from the Old Testament, and read into it a prophecy of a more recent event. Because you can control the event to be prophesied and its relationship to the Old Testament, you can usually prove in hindsight just how successful this prophecy was.