Preexistent future

There's some dispute as to whether the B-theory of time is deterministic. On the face of it, if the future already exists, then that excludes alternate timelines forking off in different directions from the present. That interval has been filled. That slot has been taken. It seems symmetrical with the accidental necessity of the past. If it's now the case that the future is already in place, then that's fixed. A fait accompli. It's already happened–just not in the present. 

That doesn't necessarily mean it's deterministic in the sense that the past causes the future. But however the future eventuated, that's now over and done with. Someone in the present has yet to experience that preexistent future, but it's already played out. 

Paul Helm has championed the B-theory of time, mainly as a model for creation by a timeless God. A fringe benefit might seem to be how it dovetails with Reformed "determinism".

However, we need to be careful about that. Reformed predestination and providence isn't based on a particular theory of time. And that might actually be inconsistent with what makes the future determinate according to Calvinism.

Assuming that there's a sense in which the B-theory of time makes the future determinate (see above), that's based on the metaphysics of time rather than a divine plan. It could be random. 

In Calvinism, the future is determinate in the way a movie plot is determinate. The director has scripted the story in his mind. It has dramatic logic. 

But that doesn't mean things had to happen in that particular sequence. Indeed, it's quite flexible. In God's imagination, the present could fork off in different directions. It's just that God picks one of those hypothetical trajectories to actualize. What makes it determinate isn't that a particular series of events had to go together, but that God chose to instantiate that particular plot. God can imagine alternate endings, but he didn't reify those counterfactuals.