Penal Code section 288.7, subdivision (a) conviction reversed because jury may have interpreted jury instruction to allow conviction based on conduct that predated the effective date of the statute. Rojas was found guilty of sexual offenses involving his stepdaughter (Pen. Code, §§ 288.5, 288.7, subds. (a), (b)). The trial court instructed the jury with a modified version of CALCRIM No. 207, which stated, in part, that it was alleged that the crimes in count 2 (Pen. Code, § 288.7, subd. (a)) and count 3 occurred on or about December 1, 2006, through August 5, 2011, and that the People were not required to prove that the crimes took place exactly during this time period, but only that they happened on a date or dates after August 12, 2005. The August 12 date was apparently related to the statute of limitations. On appeal Rojas argued that, with respect to count 2, the modified instruction resulted in an ex post facto violation insofar as it allowed jurors to convict him of violating section 288.7, subdivision (a) based on conduct that occurred before section 288.7 was enacted. Held: Reversed in part. The state and federal Constitutions prohibit ex post facto laws. “Any law that applies to events occurring before its enactment” and which disadvantages the defendant by altering the definition of criminal conduct or increasing the punishment for a crime is prohibited as ex post facto. Section 288.7 became effective on September 20, 2006. Here, the jurors were aware that the victim was born in 2002, and it is possible that they interpreted the reference to 2005 as bearing some relationship to her statements that the abuse began when she was three or four years old. As a result, the jury could have convicted Rojas based on instances of abuse that occurred before section 288.7’s effective date. This violated the state and federal ex post facto clauses. The error was not harmless.