A. The taxpayer's taxable year under this act shall be the same as his taxable year for federal income tax purposes. If, on the effective date of this act, the taxpayer's taxable year for Oklahoma income tax purposes is different than his taxable year for federal income tax purposes, the taxpayer shall file a return for the short period ending on the day for which his current taxable year for federal income tax purposes ends. If such taxpayer's taxable year for federal income tax purposes ends after his taxable year for Oklahoma income tax purposes, then the taxpayer shall file a return for such regular Oklahoma taxable year and a return for the short period ending with the federal taxable year.

The Tax Commission shall prescribe and promulgate all necessary rules and regulations for annualizing income and/or deductions for short years necessitated by the transition, if any, to the federal taxable year.

B. If a taxpayer's taxable year is changed for federal income tax purposes, his Oklahoma taxable year shall be similarly changed under the same rules applicable under the Internal Revenue Code.

C. The taxpayer's method of accounting under this act shall be the same as his method of accounting for federal income tax purposes.

D. If a taxpayer's method of accounting is changed for federal income tax purposes, such taxpayer's method for Oklahoma income tax purposes shall be similarly changed under the same rules applicable under the Internal Revenue Code.