((a)) ** Computation of taxable income** Taxable income shall be computed on the basis of the taxpayer’s taxable year.

((b)) ** Taxable year** For purposes of this subtitle, the term “taxable year” means—

((1)) the taxpayer’s annual accounting period, if it is a calendar year or a fiscal year;

((2)) the calendar year, if subsection (g) applies;

((3)) the period for which the return is made, if a return is made for a period of less than 12 months; or

((4)) in the case of a DISC filing a return for a period of at least 12 months, the period determined under subsection (h).

((c)) ** Annual accounting period** 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.

((d)) ** Calendar year** For purposes of this subtitle, the term “calendar year” means a period of 12 months ending on December 31.

((e)) ** Fiscal year** 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.

((f)) ** Election of year consisting of 52–53 weeks**

((1)) ** General rule** A taxpayer who, in keeping his books, regularly computes his income on the basis of an annual period which varies from 52 to 53 weeks and ends always on the same day of the week and ends always—

((A)) on whatever date such same day of the week last occurs in a calendar month, or

((B)) on whatever date such same day of the week falls which is nearest to the last day of a calendar month,

((2)) ** Special rules for 52–53-week year**

((A)) ** Effective dates** In any case in which the effective date or the applicability of any provision of this title is expressed in terms of taxable years beginning, including, or ending with reference to a specified date which is the first or last day of a month, a taxable year described in paragraph (1) shall (except for purposes of the computation under section 15) be treated—

((i)) as beginning with the first day of the calendar month beginning nearest to the first day of such taxable year, or

((ii)) as ending with the last day of the calendar month ending nearest to the last day of such taxable year,

((B)) ** Change in accounting period** In the case of a change from or to a taxable year described in paragraph (1)—

((i)) if such change results in a short period (within the meaning of section 443) of 359 days or more, or of less than 7 days, section 443(b) (relating to alternative tax computation) shall not apply;

((ii)) if such change results in a short period of less than 7 days, such short period shall, for purposes of this subtitle, be added to and deemed a part of the following taxable year; and

((iii)) if such change results in a short period to which subsection (b) of section 443 applies, the taxable income for such short period shall be placed on an annual basis for purposes of such subsection by multiplying the gross income for such short period (minus the deductions allowed by this chapter for the short period, but only the adjusted amount of the deductions for personal exemptions as described in section 443(c)) by 365, by dividing the result by the number of days in the short period, and the tax shall be the same part of the tax computed on the annual basis as the number of days in the short period is of 365 days.

((3)) ** Special rule for partnerships, S corporations, and personal service corporations** The Secretary may by regulation provide terms and conditions for the application of this subsection to a partnership, S corporation, or personal service corporation (within the meaning of section 441(i)(2)).

((4)) ** Regulations** The Secretary shall prescribe such regulations as he deems necessary for the application of this subsection.

((g)) ** No books kept; no accounting period** Except as provided in section 443 (relating to returns for periods of less than 12 months), the taxpayer’s taxable year shall be the calendar year if—

((1)) the taxpayer keeps no books;

((2)) the taxpayer does not have an annual accounting period; or

((3)) the taxpayer has an annual accounting period, but such period does not qualify as a fiscal year.

((h)) ** Taxable year of DISC’s**

((1)) ** In general** For purposes of this subtitle, the taxable year of any DISC shall be the taxable year of that shareholder (or group of shareholders with the same 12-month taxable year) who has the highest percentage of voting power.

((2)) ** Special rule where more than one shareholder (or group) has highest percentage** 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).

((3)) ** Subsequent changes of ownership** 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.

((4)) ** Voting power determined** For purposes of this subsection, voting power shall be determined on the basis of total combined voting power of all classes of stock of the corporation entitled to vote.

((i)) ** Taxable year of personal service corporations**

((1)) ** In general** 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.

((2)) ** Personal service corporation** For purposes of this subsection, the term “personal service corporation” has the meaning given such term by section 269A(b)(1), except that section 269A(b)(2) shall be applied—

((A)) by substituting “any” for “more than 10 percent”, and

((B)) by substituting “any” for “50 percent or more in value” in section 318(a)(2)(C).