Update: My comment of Jan. 19, 2013 may contain a satisfactory answer.
In an earlier post, I argued that asking why it's 2013 presently forces the A-theorist to deny the Principle of Sufficient Reason (PSR). Let me expand on that argument. Here's a thought about my main argument.
Suppose the PSR is true. With the typical A-theorist, consider the proposition q that it is 2013 presently to be a contingent proposition that changes in truth value—it was false a couple of weeks ago and will be false again in a year but is now true. Let p be the ultimate explanation of q. For if PSR is true, there are ultimate explanations. Cf. my PSR book. Then either p is a proposition that is true at all times or it is a proposition only true at some times.
But it is incredible what proposition that is true at all times (in 2012, in 94994 BC, etc.) would explain why it is 2013 presently.
But if p is true at some but not all times, then p can't be an ultimate explanation, because it is unexplained why p is presently true, rather than this being one of those times where p is false.
So, either way, we get a contradiction.
The typical B-theorist, on the other hand, will say that all true propositions are eternally true, so q, if there is such a proposition at all, is eternally true, and there is no difficulty about explaining it with another eternally true proposition.
Objection: But Pruss has argued at length (say, in the PSR book or here) that it is possible to explain a contingent proposition with a necessary one. So why can't one explain a non-eternal truth with an eternal one, explaining q with an eternal truth p?
Response: I just don't see any plausible way to do it, that's all. I wish I had something more rigorous to say here.