Cal. Revenue and Taxation Code § 45153.5

(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 fee was due until the date of payment, if all of the following occur: (1) The payment of the fee was made one business day after the date the fee was due. (2) The person was granted relief from all penalties that applied to that fee payment. (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 does not apply to any payment made pursuant to a deficiency determination, a determination where a return has not been filed, or a jeopardy determination issued by the department. (e) This section only applies to electronic payments of fees.