California Code, Financial Code - FIN § 22452

(a) The actual daily unpaid balances of the open-end account in the billing cycle for which the charge is made, in which case one-thirtieth of the monthly rate may be charged for each day the unpaid balance is outstanding.

(b) The average daily unpaid balance of the open-end account in the billing cycle for which the charge is made, which is the sum of the amount unpaid each day during that cycle divided by the number of days in that cycle.  The amount unpaid on a day is determined by adding to any balance unpaid as of the beginning of that day all advances and other debits and deducting all payments and other credits made or received as of that day.  The billing cycle shall be monthly.  A billing cycle is monthly if the closing date of the cycle is the same date each month or does not vary by more than four days from the regular date.

This section does not apply to any open-end loan of a bona fide principal amount of ten thousand dollars ($10,000) or more as determined in accordance with Section 22467.