Reduction is not a decision procedure
There are some arguments against the reducibility of tensed propositions to tenseless propositions about times and things at times. But I've never seen the following argument:
The reductionist claims that there are other times and that things have all kinds of properties at those times. Clearly, it would be circular to say that there are exactly those times that once existed or will exist, and that x has F at some past time iff x once was F. The reductionist must not use tensed statements in specifying exactly what times there are and what things instantiate which properties at those times. But it seems hopeless to find a completely tenseless, general, and yet accurate rule.
This is silly, because a reduction is not the same things as a decision procedure. Of course, if you reduce A-facts to B-facts, complete knowledge of B-facts must in principle suffice to deduce all A-facts. But specifying all the B-facts is in no way part of the reduction.
Isn't it puzzling that this silly kind of argument keeps being brought forward against Lewis' reduction of modal facts to facts about possibilia (e.g. in Lycan, "Two -- no, three -- concepts of possible worlds", Proceedings of the Aristotelian Society (91): 1991; Divers and Melia, "The analytic limit of genuine modal realism", Mind (111): 2002)?