27 C.F.R. § 24.277 – Date of mailing or delivering of returns.

(a) When the proprietor sends the Excise Tax Return, TTB F 5000.24, with or without remittance, by United States mail, the official postmark of the United States Postal Service stamped on the cover of the envelope in which the return was mailed is considered the date of delivery of the tax return and, if accompanied, the date of delivery of the remittance. When the postmark on the cover is illegible, it is the proprietor's responsibility to prove when the postmark was made.
(b) When the proprietor sends the tax return by registered mail or by certified mail, the date of registry or the date of the postmark on the sender's receipt of certified mail, as the case may be, is treated at the date of delivery of the tax return and, if accompanied, the date of delivery of the remittance.
(August 16, 1954, ch. 736, 68A Stat. 775, as amended, 777, as amended, 391, as amended (26 U.S.C. 6301, 6311, 6302))
(Approved by the Office of Management and Budget under control numbers 1512-0467 and 1512-0492)