Interest on unpaid tax demands is charged monthly, with limited exemption for penalty-related demands and specified orders. Interest is made chargeable on unpaid amounts specified in a notice of demand under section 289 when payment is not made within the prescribed period. The assessee becomes liable to simple interest at 1% for every month or part of a month, calculated from the day immediately after the expiry of the payment period until the date of actual payment. No interest is chargeable on penalty-related demands under section 439 up to the relevant order dates specified for section 359 and section 363 cases.