What is the test for an ex post facto violation of a sentence in a sexual assault case?

"[I]t is the prosecution's responsibility to prove to the jury that the charged offenses occurred on or after the effective date of the statute providing for the defendant's punishment. When the evidence at trial does not establish that fact, the defendant is entitled to be sentenced under the formerly applicable statutes even if he raised no objection in the trial court." (People v. Hiscox (2006) 136 Cal.App.4th 253, 256.)

An ex post facto violation resulting in an unauthorized sentence may be raised on appeal even if the defendant failed to object below and may be corrected at any time. (People v. Dotson (1997) 16 Cal.4th 547, 554, fn. 6.)