Whether the A-series or B-series theorists have it right - that is, whether or not tensed utterances can be reduced to tenseless propositions without a loss of meaning - for every tensed utterance, there is a corresponding tenseless proposition. For example, on the assumption that the tensed utterance “I am presently typing [now]” is true, the corresponding tenseless utterance “I type on 4/29/2013” would also be true because either 4/29/2013 is the ever-flowing “now” or “the present” (A-series) or because the utterance of the tensed expression is simultaneous in the causal chain with the change in date to 4/29/2013 (B-series).
If we assume God is in time, as some Arminians but especially most Molinists are wont to argue, then the A-series theory is correct. But in that case, on Calvinism, God would know this and every other tenseless truth which corresponds to a tensed truth because He has predetermined all things. That is, even if God is in time, “the future” is not open to Him. God may “take on new knowledge” in the sense that His affirmation of tensed propositions would vary according to their change in truth-value, but God would know beforehand what that He will take on “new knowledge” as well as what the content of this “new knowledge” is.
On the other hand, for the Arminian or Molinist, God’s knowledge of tenseless truths relating to His creatures who act “freely” is contingent on their fulfillment in time. So the future is open to God, at least in this respect. Molinists will debate this, but I think the grounding objection is sound (link). When we say God “takes on new knowledge” in this case, then, we mean something more than “God believes to be true what He knew He would believe to be true.” For on Arminianism and Molinism, God didn’t know what He would believe with respect to our allegedly free choices before we made them. He couldn’t have, or else they weren't really “free” in the sense both Arminians and Molinists want to preserve. That is, if God knew beforehand what they would choose, then men couldn’t have exercised their wills according to two or more courses of action.