Contributions unpaid on the date on which they are due and payable, as prescribed by the department, shall bear interest at one-twelfth (1/12) of the annual rate determined by the secretary of revenue under section 806 of the act of April 9, 1929 (P.L. 343, no.176), known as "the fiscal code," per month or fraction of a month, or at the rate of one per centum (1%) per month or fraction of a month, whichever is greater, from the date they become due until paid.
43 P.S. § 788