(a) General rule. -- For purposes of the tax imposed by this article, a taxpayer's taxable year shall be the same as the taxpayer's taxable year for federal income tax purposes.
(b) Change of taxable year. -- If a taxpayer's taxable year is changed for federal income tax purposes, the taxpayer's taxable year for purposes of this article shall be similarly changed. The taxpayer shall provide a copy of the authorization for such change from the Internal Revenue Service, with its annual return for the taxable year filed under this article.
(c) Cash methods of accounting required. -- A taxpayer's method of accounting under this article shall be the cash method of accounting, whether or not taxpayer uses the cash method of accounting for federal income tax purposes.