Cal. Fam. Code § 3653

(a) An order modifying or terminating a support order may be made retroactive to the date of the filing of the notice of motion or order to show cause to modify or terminate, or to any subsequent date, except as provided in subdivision (b) or by federal law (42 U.S.C. Sec. 666(a)(9)).
(b) If an order modifying or terminating a support order is entered due to the unemployment of either the support obligor or the support obligee, the order shall be made retroactive to the later of the date of the service on the opposing party of the notice of motion or order to show cause to modify or terminate or the date of unemployment, subject to the notice requirements of federal law (42 U.S.C. Sec. 666(a)(9)), unless the court finds good cause not to make the order retroactive and states its reasons on the record.
(c) If an order modifying or terminating a support order is entered due to a change in income resulting from the activation to United States military service or National Guard duty and deployment out of state for either the support obligor or the support obligee, the order shall be made retroactive to the later of the date of the service on the opposing party of the notice of activation, notice of motion, order to show cause to modify or terminate, or the date of activation, subject to the notice requirements of federal law (42 U.S.C. Sec. 666(a)(9)), unless the court finds good cause not to make the order retroactive and states its reasons on the record. Good cause shall include, but not be limited to, a finding by the court that the delay in seeking the modification was not reasonable under the circumstances faced by the service member.
(d) If an order decreasing or terminating a support order is entered retroactively pursuant to this section, the support obligor may be entitled to, and the support obligee may be ordered to repay, according to the terms specified in the order, any amounts previously paid by the support obligor pursuant to the prior order that are in excess of the amounts due pursuant to the retroactive order. The court may order that the repayment by the support obligee shall be made over any period of time and in any manner, including, but not limited to, by an offset against future support payments or wage assignment, as the court deems just and reasonable. In determining whether to order a repayment, and in establishing the terms of repayment, the court shall consider all of the following factors:
1. (1) The amount to be repaid.
2. (2) The duration of the support order prior to modification or termination.
3. (3) The financial impact on the support obligee of any particular method of repayment such as an offset against future support payments or wage assignment.
4. (4) Any other facts or circumstances that the court deems relevant.