California Family Code Section 3651
A child or spousal support order or judgment can be modified or terminated only prospectively and may be made retroactive only to the date a motion to modify was filed. (Fam. Code, 3651, subd. (c); 3653, subd. (a).)
Accrued arrearages are treated like a money judgment. (In re Marriage of Everett (1990) 220 Cal.App.3d 846, 854.)
Generally, a first party owing money to a second party may offset the amount owed by any amount the second party owes to the first party. (Keith G. v. Suzanne H. (1998) 62 Cal.App.4th 853, 859.)
But this general rule does not necessarily apply to child or spousal support arrearages.
The application of setoff to support arrearages was discussed by the California Supreme Court in Keck v. Keck (1933) 219 Cal. 316, 320 (Keck) and the First Appellate District, Division One, in Williams v. Williams (1970) 8 Cal.App.3d 636 (Williams).