I have been told that the following are the same:
It has been twenty years since I last saw her.
It is twenty years since I have seen her.
But I somehow do not understand why; usually the present perfect is somehow related to present ("I have not seen her recently"). I would tend to say that he has been seeing her since then (twenty years ago).
Also why does the first one contain "It has been" and the other one "It is". Does it matter?