Interest

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