Cal. Rev. & Tax. Code § 13516

In a case not involving a false or fraudulent return or failure to file a return, if the Controller determines at any time after the tax is due, but not later than four years after the return is filed, that the tax disclosed in any return required to be filed by this part is less than the tax disclosed by his or her examination, a deficiency shall be determined. That determination may also be made within such time after the expiration of that four-year period as may be agreed upon in writing between the Controller and the personal representative.

For purposes of this section, a return filed before the last day prescribed by law for filing that return shall be considered as filed on that last day.