Cal. Revenue and Taxation Code § 6591.6

(a) If the department finds, taking into account all facts and circumstances, that it is inequitable to compute interest at the modified adjusted rate per month or fraction thereof, as defined in subdivision (b) of Section 6591.5, interest shall be computed at the modified adjusted daily rate from the date on which the tax or prepayment was due until the date of payment, if all of the following occur: (1) The payment of tax or the prepayment was made one business day after the date the tax or prepayment was due. (2) The person was granted relief from all penalties that applied to that payment of tax or prepayment. (3) The person files a request for an adjustment. (b) For purposes of this section, “modified adjusted daily rate” means the modified adjusted rate per annum as defined in subdivision (a) of Section 6591.5 determined on a daily basis by dividing the modified adjusted rate per annum by 365. (c) For purposes of this section, “business day” means any day other than a Saturday, Sunday, or any day designated as a state holiday. (d) This section shall not apply to any payment made pursuant to a deficiency determination, a determination where no return has been filed, or a jeopardy determination issued by the department. (e) This section shall only apply to electronic payments or prepayments of taxes.