Cal. Gov. Code § 31589.1
There is hereby established for accounting purposes in the County Employees Retirement Law of 1937 the following procedure for treating a trade of bonds for similar bonds. Any loss or gain attributable to a trade of a like bond in the portfolio of any retirement system adopted pursuant to this chapter may be amortized over the life of the bond traded out by adding to or subtracting from the discount or premium attributable to the bond traded in. Like bonds for purposes of this section are considered to be bonds which will mature within seven years of the life of the bond traded out. Bonds to be traded must be of the first four grades. The fact that one bond may be a debenture and another a mortgage bond, or that the bonds may have different rates of return, shall not keep them from being like bonds.
This section shall not be operative in any county until such time as the board shall, by resolution adopted by majority vote, make the provisions of this section applicable in such county.
Ca. Gov. Code § 31589.1