Cal. Unemp. Ins. Code § 1129

The amount of each assessment shall bear interest at the adjusted annual rate and by the method established pursuant to Section 19521 of the Revenue and Taxation Code from and after the last day of the month following the close of the calendar quarter, or from and after the 15th day of the month following the close of the calendar month, for which the contributions should have been returned until the date of payment.