Exception to final installment.

If a taxpayer files a return for a calendar year on or before January 31 of the succeeding calendar year (or if a taxpayer on a fiscal year basis files a return on or before the last day of the first month immediately succeeding the close of such fiscal year) and pays in full at the time of such filing the amount computed on the return as payable, then, if estimated taxes are not required to be paid on or before the 15th day of the 9th month of the taxable year but are required to be paid on or before January 15 of the succeeding taxable year (or the date corresponding thereto in the case of a fiscal year), such return shall be considered as such payment.