deposits in a fund which are made within 60 days after the date of execution of the agreement, or on or before the due date, with extensions thereof, for the filing of his Federal income tax return for such taxable year or years, whichever date shall be later, shall be deemed to have been made on the date of the actual deposit or as of the close of business of the last regular business day of each such taxable year or years to which such deposits relate, whichever day is earlier.
26 C.F.R. §3.3