RTC §50140: (a) Except as provided in subdivision (b), the board shall not approve a refund three years after the due date of the payment for the period for which the overpayment was made, or, with respect to det

(a) Except as provided in subdivision (b), the board shall not approve a refund three years after the due date of the payment for the period for which the overpayment was made, or, with respect to determinations made under Article 2 (commencing with Section 50113) of Chapter 3, after six months from the date the determinations have become final, or after six months from the date of overpayment, whichever period expires later, unless a claim therefor is filed with the board within that period. The board shall not approve a credit after the expiration of that period, unless a claim for credit is filed with the board within that period. (b) A refund may be approved by the board for any period for which a waiver is given under Section 50113.2 if a claim is filed with the board before the expiration of the period agreed upon. (c) Every claim for refund or credit shall be in writing and shall state the specific grounds upon which the claim is founded.