Interest

If any proprietor fails to pay tax due as required under the provisions of this Act or the rules, he shall, in addition to tax due, be liable to pay simple interest on the amount so due at the rate of one and half per centum per month from the date immediately following the last date for payment of tax, for a period of one month, and at the rate of two per centum per month thereafter so long as he continues to make default in such payment.