Interest

There shall be charged, levied and paid simple interest at the rate of two per centum per month on any outstanding sum of tax or penalty due under this Act except the penalty imposed under section 19 from the day such sum first becomes due to the day it is paid notwithstanding any order of permitting payment by installment of stay of recovery if the outstanding sum is ultimately found due against the dealer: Provided that for the purpose of calculation of interest, a period of fifteen day or more shall be deemed to be one month and the amount of fifty rupees or more shall be deemed to be one hundred rupees and a period of less than fifteen days and an amount of less than fifty rupees shall be ignored.