Per CCP Section 685.010, Interest on Child Support and Spousal Support Judgments Accrues at Ten Percent Per Annum From the Date Each Installment is Due
Interest runs from the due date of each monthly payment (or if the order requires payment on the first and fifteenth day of the month, from those dates). So, imagine an order on January 1, 2010 at $200/month. Nothing is paid. The support recipient files a motion on December 23, 2016, seeking all unpaid child or spousal support plus interest @ 10%. The interest they are entitled to must be calculated from the date each installment was due. This would be seven years at $2,400 per year, or $16,800. But the interest at 10% is not on the entire $16,800; it has to be separately calculated on the oldest months to the most recent. This is why family law attorneys must use programs like the Xspouse Executioner, which automatically figures out the interest. If you are seeking a support order and interest, you need to print out the Executioner math and attach it to your application!
CALIFORNIA FAMILY CODE
DEFINITIONS OF WHEN 10% INTEREST BEGINS ON UNPAID SUPPORT ORDERS
Family Code Section 155
"Support order" means a judgment or order of support in favor of an obligee, whether temporary or final, or subject to modification, termination, or remission, regardless of the kind of action or proceeding in which it is entered. For the purposes of Section 685.020 of the Code of Civil Procedure, only the initial support order, whether temporary or final, whether or not the order is contained in a judgment, shall be considered an installment judgment. No support order or other order or notice issued, which sets forth the amount of support owed for prior periods of time or establishes a periodic payment to liquidate the support owed for prior periods, shall be considered a money judgment for purposes of subdivision (b) of Section 685.020 of the Code of Civil Procedure.