 On a B-theory of time an event’s being past, present or future is simply a relational matter; it is simply a matter of its occurring at a time which is earlier than, or simultaneous with, or later than some time which is taken as the reference point – the time at which I am having this thought, say. On an A-theory, on the other hand, that this time (or this event) is now is an absolute (if temporary) fact about it. But, in addition, a B-theory will typically hold that all times are equally real, whereas, typically, an A-theory will hold either that only the present and the past are real (type 1), or that only the present is real (type 2). So, on a B-theory, there can be future facts to make propositions about the future true; but, on an A-theory of type 1, a proposition about the future can be made true only by present and past facts together with timeless facts; and, on an A-theory of type 2, propositions both about the future and about the past, can be made true only by present facts together with timeless facts. And, evidently, on either type of A-theory a proposition may be true at one time, but not true at another, depending on what facts exist at the time. (For a fuller description of the A-theory and B-theory, see the Section 5 of the entry on  time.)  