What is the test for establishing that the date of an alleged sexual assault occurred prior to the effective date of the sexual assault statute?

Cal.App.4th 253, 256.) If the dates of the offenses as charged include a time period predating the effective date of the statute, and if the jury makes no finding that the offense occurred on or after the effective date, the verdicts cannot be deemed sufficient to establish the date of the offenses unless the evidence leaves no reasonable doubt that they occurred on or after the effective date. (Id. at p. 261, citing Chapman v. California (1967) 386 U.S. 18, 24 (Chapman).)