Interest for late return accrues monthly on determined interest-tax liability, reduced by interest-tax paid in advance. Interest for default attaches where a return of chargeable interest is furnished after the due date or not furnished: simple interest at the prescribed monthly rate is payable from the day after the due date until filing or completion of assessment, computed on interest tax determined under the assessment provisions and reduced by interest tax paid in advance; late filings in response to post assessment notices attract interest only on the increase in interest tax on reassessment, and appellate or rectification orders increase or reduce the interest accordingly with corresponding demand or refund mechanisms.