1.) Take Aristotle’s sea battle. Either there is one (S) or not (~S)
2.) Restricting ourselves to the present moment, the conditional is necessary and, necessarily, one option is necessary and the other impossible.
3.) In a moment considered as later, the conditional is necessary but, necessarily, both options are contingent.
4.) The present can be considered either within itself (2) or as a moment later than the past (3). Similar things occur with the past considered in itself (2) or relative to what is even more remote in the past (3).
This breaks down in the relation between the future and future-perfect, where there are earlier-later relationships but all parts of the conditional are contingent. It is true that a sea battle will either have happened or not by the time you count to ten, but reaching ten or not is the same sort of thing as the sea battle. All we are doing is taking a future thing as given, not discovering it as necessary.
6.) So the temporal definition of modalities is incompatible with B-theory, since earlier-later divisions do not suffice to account for modality defined temporally. The incompatibility is particularly strong: assuming the adequacy of B-theory, we have no idea what to say about the contingency or necessity of past, present, or future events.
7.) But this seems to require that, if I see a sea battle happening now then, for all I know, it is just as possible that it is not a sea battle. B-Theory goes and time is real.