Gov. Code, § 20163

(a) If more or less than the correct amount of contribution required of members, the state, or any contracting agency, is paid, proper adjustment shall be made in connection with subsequent payments, or the adjustments may be made by direct cash payments between the member, state, or contracting agency concerned and the board or by adjustment of the employer’s rate of contribution. Adjustments to correct any other errors in payments to or by the board, including adjustments of contributions, with interest, that are found to be erroneous as the result of corrections of dates of birth, may be made in the same manner. Adjustments to correct overpayment of a retirement allowance may also be made by adjusting the allowance so that the retired person or the retired person and his or her beneficiary, as the case may be, will receive the actuarial equivalent of the allowance to which the member is entitled. Losses or gains resulting from error in amounts within the limits set by the Department of General Services for automatic writeoff, and losses or gains in greater amounts specifically approved for writeoff by the Department of General Services, shall be debited or credited, as the case may be, to the reserve against deficiencies in interest earned in other years, losses under investments, and other contingencies.

(b) No adjustment shall be made because less than the correct amount of normal contributions was paid by a member if the board finds that the error was not known to the member and was not the result of erroneous information provided by him or her to this system or to his or her employer. The failure to adjust shall not preclude action under Section 20160 correcting the date upon which the person became a member.

(c) The actuarial equivalent under this section shall be computed on the basis of the mortality tables and actuarial interest rate in effect under this system on December 1, 1970, for retirements effective through December 31, 1979. Commencing with retirements effective January 1, 1980, and at corresponding 10-year intervals thereafter, or more frequently at the board’s discretion, the board shall change the basis for calculating actuarial equivalents under this article to agree with the interest rate and mortality tables in effect at the commencement of each 10-year or succeeding interval.