701—10.97 (422) Interest on tax due. All tax which has not been paid on or before the last day of the ninth month following the death of the individual whose death is the event imposing the federal generation skipping transfer tax draws interest at the rate prescribed by Iowa Code section 421.7, to be computed on a monthly basis with each fraction of a month counted as a full month. See rule 701—10.2(421) for the interest rate to use for a specific calendar year. Interest applies equally to tax that is delinquent and tax that is due under an extension of time to pay.