(a) If any amount of tax imposed by Part II of this article is not paid on or before the last date prescribed for payment, interest on such amount at the rate established pursuant tosection 806 of the act of April 9, 1929 (P.L. 343, No. 176), known as "The Fiscal Code," shall be paid for the period from such last date to the date paid. The last date prescribed for payment shall be determined without regard to any extension of time for filing the return. This section shall not apply to any failure to pay estimated tax.(b) If any amount of tax required to be withheld by an employer and paid to the department under Part VII of this article is not paid by the due date prescribed under section 319, interest on the amount at the rate established undersection 806 of "The Fiscal Code" shall be paid from that date for the period of underpayment.1971, March 4, P.L. 6, No. 2, art. III, § 351, added 1971, Aug. 31, P.L. 362, No. 93, § 4. Amended 1977, Dec. 21, P.L. 330, No. 98, § 2, imd. effective; 1984, June 29, P.L. 445, No. 94, § 1, imd. effective.