Interest on delayed duty payments now mandatory at government fixed rates, calculated from due month with narrow exceptions for Board orders. Section 28AA imposes statutory interest on delayed customs duty payments regardless of court or tribunal orders, applicable to voluntary payments and those made after determination. The Central Government fixes an annual rate within a prescribed band, and interest runs from the first day of the month after duty became payable or from the date of an erroneous refund until payment. Interest is not payable where duty follows a Board order and the amount is paid in full voluntarily within a limited period without reserving appeal rights.