Code of Federal Regulations Title 26. Internal Revenue § 26.1.6164–5 Period of extension

(a) On the last day of the month in which falls the last date prescribed by law (including any extension of time granted the taxpayer) for the filing of the return for the taxable year of the expected net operating loss;  or

(b) If an application for a tentative carryback adjustment provided in section 6411 with respect to such loss is filed before the expiration of the period specified in paragraph (a) of this section, on the date on which notice is mailed by registered mail prior to September 3, 1958, and by either registered or certified mail on and after September 3, 1958, to the taxpayer that such application is allowed or disallowed in whole or in part.