The board for good cause may extend for not to exceed one month the time for making any return or paying any tax required under this part. The extension may be granted at any time; provided, a request therefor is filed with the board within or prior to the period for which the extension may be granted.
Any user to whom an 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 on which the tax would have been due without the extension to the date of payment.