Taxable income shall be computed on the basis of the taxpayer’s taxable year.
For purposes of this subtitle, the term “annual accounting period” means the annual period on the basis of which the taxpayer regularly computes his income in keeping his books.
For purposes of this subtitle, the term “calendar year” means a period of 12 months ending on December 31.
For purposes of this subtitle, the term “fiscal year” means a period of 12 months ending on the last day of any month other than December. In the case of any taxpayer who has made the election provided by subsection (f) the term means the annual period (varying from 52 to 53 weeks) so elected.
If 2 or more shareholders (or groups) have the highest percentage of voting power under paragraph (1), the taxable year of the DISC shall be the same 12-month period as that of any such shareholder (or group).
The Secretary shall prescribe regulations under which paragraphs (1) and (2) shall apply to a change of ownership of a corporation after the taxable year of the corporation has been determined under paragraph (1) or (2) only if such change is a substantial change of ownership.
For purposes of this subtitle, the taxable year of any personal service corporation shall be the calendar year unless the corporation establishes, to the satisfaction of the Secretary, a business purpose for having a different period for its taxable year. For purposes of this paragraph, any deferral of income to shareholders shall not be treated as a business purpose.