What is the test for a jury to determine when a defendant committed a criminal offence prior to the date of his conviction?

As the court stated in People v. Hiscox, supra, 136 Cal.App.4th at page 261: "It would be inappropriate for us to review the record and select among acts that occurred before and after that date, or to infer that certain acts probably occurred after that date. Hiscox has a constitutional right to be sentenced under the terms of the laws in effect when he committed his offenses. For a court to hypothesize which acts the jury may have based its verdicts on, or what dates might be attached to certain acts based on ambiguous evidence, would amount to 'judicial impingement upon the traditional role of the jury.' (Blakely v. Washington[ (2004) 542 U.S. 296,] at p. 309.)"

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