(a) In general. For purposes of subchapter S of chapter 1 of the Internal Revenue Code (Code) and this section, the term post-termination transition period means—

(1) The period beginning on the day after the last day of the corporation's last taxable year as an S corporation and ending on the later of—

(i) The day which is 1 year after such last day; or

(ii) The due date for filing the return for the last taxable year as an S corporation (including extensions);

(2) The 120-day period beginning on the date of any determination pursuant to an audit of the taxpayer which follows the termination of the corporation's election and which adjusts a subchapter S item of income, loss, or deduction of the corporation arising during the S period (as defined in section 1368(e)(2)); and

(3) The 120-day period beginning on the date of a determination that the corporation's election under section 1362(a) had terminated for a previous taxable year.

(b) Special rules for post-termination transition period. Pursuant to section 1377(b)(1) and paragraph (a)(1) of this section, a post-termination transition period arises the day after the last day that an S corporation was in existence if a C corporation acquires the assets of the S corporation in a transaction to which section 381(a)(2) applies. However, if an S corporation acquires the assets of another S corporation in a transaction to which section 381(a)(2) applies, a post-termination transition period does not arise. (See § 1.1368-2(d)(2) for the treatment of the acquisition of the assets of an S corporation by another S corporation in a transaction to which section 381(a)(2) applies.)

(c) Determination defined. For purposes of section 1377(b)(1) and paragraph (a) of this section, the term determination means—

(1) A determination as defined in section 1313(a);

(2) A written agreement between the corporation and the Commissioner (including a statement acknowledging that the corporation's election to be an S corporation terminated under section 1362(d)) that the corporation failed to qualify as an S corporation;

(3) For a corporation subject to the audit and assessment provisions of subchapter C of chapter 63 of subtitle A of the Code, the expiration of the period specified in section 6226 for filing a petition for readjustment of a final S corporation administrative adjustment finding that the corporation failed to qualify as an S corporation, provided that no petition was timely filed before the expiration of the period; and

(4) For a corporation not subject to the audit and assessment provisions of subchapter C of chapter 63 of subtitle A of the Code, the expiration of the period for filing a petition under section 6213 for the shareholder's taxable year for which the Commissioner has made a finding that the corporation failed to qualify as an S corporation, provided that no petition was timely filed before the expiration of the period.

(d) Date a determination becomes effective—(1) Determination under section 1313(a). A determination under paragraph (c)(1) of this section becomes effective on the date prescribed in section 1313 and the regulations thereunder.

(2) Written agreement. A determination under paragraph (c)(2) of this section becomes effective when it is signed by the district director having jurisdiction over the corporation (or by another Service official to whom authority to sign the agreement is delegated) and by an officer of the corporation authorized to sign on its behalf. Neither the request for a written agreement nor the terms of the written agreement suspend the running of any statute of limitations.

(3) Implied agreement. A determination under paragraph (c) (3) or (4) of this section becomes effective on the day after the date of expiration of the period specified under section 6226 or 6213, respectively.