Interest at the rate of 0.75 % per month, for each month or fraction thereof for which any amount of tax imposed by this article is not paid on or before the last day prescribed for payment, shall be imposed 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, however, this section shall not apply to any failure to pay estimated tax.
61 Pa. Code § 119.19