Addition to the tax
(a) In case of failure to file any return required under authority of chapter 21 of this title (other than subchapter III thereof), of chapter 1 of this title (relating to inheritance taxes), of chapter 3 of this title (relating to certain excise taxes and gross receipts tax), or of chapter 5 of this title (relating to gasoline taxes), on the date prescribed therefor (determined with regard to any extension of time for filing), unless it is shown that such failure is due to reasonable cause and not due to willful neglect, there shall be added to the amount required to be shown as tax on such return 5 percent of the amount of such tax if the failure is for not more than 1 month, with an additional 5 percent for each additional month or fraction thereof during which such failure continues, not exceeding 25 percent in the aggregate.
Penalty imposed on net amount due
(b) For purposes of subsection (a) of this section the amount of tax required to be shown on the return shall be reduced by the amount of any part of the tax which is paid on or before the date prescribed for payment of the tax and by the amount of any credit against the tax which may be claimed upon the return.