Cal. Rev. & Tax. Code § 32253

The board for good cause may extend for not to exceed one month the time for making any report or paying any tax. Any person to whom an extension is granted and who pays the tax within the period for which the extension is granted shall pay, in addition to the tax, interest at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the date the tax would have been due without the extension to the date of payment.