26 CFR § 301.6501(b)-1 - Time return deemed filed for purposes of determining limitations.
(a) Early return. Any return, other than a return of tax referred to in paragraph (b) of this section, filed before the last day prescribed by law or regulations for the filing thereof (determined without regard to any extension of time for filing) shall be considered as filed on such last day.
(b) Returns of social security tax and of income tax withholding. If a return on or after November 13, 1966, of tax imposed by chapter 3 of the Code (relating to withholding of tax on nonresident aliens and foreign corporations and tax-free covenant bonds), or if a return of tax imposed by chapter 21 of the Code (relating to the Federal Insurance Contributions Act) or by chapter 24 of the Code (relating to collection of income tax at source on wages), for any period ending with or within a calendar year is filed before April 15 of the succeeding calendar year, such return shall be deemed filed on April 15 of such succeeding calendar year. For example, if quarterly returns of the tax imposed by chapter 24 of the Code are filed for the four quarters of 1955 on April 30, July 31, and October 31, 1955, and on January 31, 1956, the period of limitation for assessment with respect to the tax required to be reported on such return is measured from April 15, 1956. However, if any of such returns is filed after April 15, 1956, the period of limitation for assessment of the tax required to be reported on that return is measured from the date it is in fact filed.
(c) Returns executed by district directors or other internal revenue officers. The execution of a return by a district director or other authorized internal revenue officer or employee under the authority of section 6020(b) shall not start the running of the statutory period of limitations on assessment and collection.