California Revenue & Taxation Code
Cal. Rev. & Tax. Code § 40083
Official textleginfo.legislature.ca.gov
The amount of the determination, exclusive of penalties, shall bear interest at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the last day of the month following the quarterly period for which the amount or any portion thereof should have been returned until the date of payment.
Source: view the official text