Interest on delayed customs duty applies until payment, with a limited exception for prompt voluntary compliance after Board directions. Interest on delayed payment of customs duty is payable by a person liable under section 28, in addition to the duty itself, whether payment is voluntary or after determination of liability. The rate is fixed by the Central Government within the statutory range, and interest runs from the first day of the month succeeding the month in which duty ought to have been paid, or from the date of an erroneous refund, until payment. No interest is payable where duty is paid in full within forty-five days of a Board order under section 151A without reserving any right to appeal.