Cal. Rev. & Tax. Code § 24409

The election provided by Section 24407 may be made for any taxable year beginning after December 31, 1960, but only if made not later than the time prescribed by law for filing the return for that taxable year (including extensions thereof). The period so elected shall be adhered to in computing the income of the corporation for the taxable year for which the election is made and all subsequent taxable years. The election shall apply only with respect to the expenditures paid or incurred on or after June 23, 1961.