Cal. PCC § 10320.5

(a) Commencing January 1, 1992, all state agencies subject to this chapter that enter into installment purchase or lease-purchase contracts shall make periodic payments, which shall include interest computed from a date no later than the acceptance date of the goods purchased pursuant to the contract. However, if the contract requires an acceptance test, interest shall be computed from a date no later than the first day of the successful acceptance test period. Unless otherwise provided for in the contract, periodic payments shall commence upon acceptance of the goods or, if the contract requires an acceptance test, as of the first day of the successful acceptance test period. Late charges shall accrue for any periodic payment not made to the contractor or its assigns from either the payment date provided in the contract or 60 days following the receipt of a valid invoice for the periodic payment, whichever is later. However, in the event any invoice is received prior to the acceptance date, the receipt date of the invoice shall be construed to be the acceptance date. Late charges under this section shall be assessed using the interest rate as specified in Section 927.6 of the Government Code.

(b) The department is authorized to refinance installment purchase contracts when, in the determination of the department, it is financially beneficial to the state to do so.