When, by reason of the assignment or reassignment of a renewal registration date by the director, the registration year is less than, or more than, 12 months, the fee due for that renewal shall be decreased or increased by one-twelfth of the annual fee for each month of the period less than, or in excess of, 12 months.
Source:
Section 9559.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH§ionNum=9559.5..