California Code, Government Code - GOV § 926.10

Any public entity as defined by Section 811.2 having a liquidated claim against any other public entity based on contract or statute of the State of California, or any person having such a claim against a public agency, shall be entitled to interest commencing the 61st day after such public entity or person files a liquidated claim known or agreed to be valid when filed pursuant to such statute or contract, and such claim is due and payable. Interest shall be 6 percent per annum.