I've often wondered about that. I really don't know what I think is most likely. Most often I lean toward thinking that all time exists - but the we only conciously experience one moment at a time. Just like the whole globe exists, but we only exist in one part of it at a time. On the globe, via various means of communication lets us be aware of places far from us on the globe. Perhaps, in a similar way, we can be aware of other moments in time even while we're not in them. Memory lets us do that with the past, perhaps something can let us do that with the future.