What is the legal test for a motion to waive the statute of limitations in a motion for a PRA defense?

After analyzing the types of defenses listed in 12(b)(1) and (2), the court concluded that the PRA defense "is a permissive pretrial matter that the defendant may, but need not, raise by motion before trial." Id. at 1098. It likened the PRA defense to former jeopardy, former conviction, statute of limitations and immunity, all "in the nature of affirmative defenses," and all permissibly raised either before trial or at trial. Id. See also United States v. DeTar, 832 F.2d 1110, 1114 (9th Cir.1987) (affirmative defense of statute of limitations is waived unless raised before or at trial).