Interest under section 234B of the Act, even though mandatory, cannot be levied if the addition to income is made on account of a retrospective amendment. In this case, the addition was made during assessment on account of excess credit period granted to an associated enterprise. The provisions which regard excess credit period granted to an associated enterprise as an ‘international transaction’ were introduced by way of an amendment to section 92B of the Act by Finance Act, 2012 with retrospective effect from 01.04.2002. Therefore, since at the time of filing of the return of income for AY 2008-09 on 27.09.2008, these provisions did not exist, the assessee could not be regarded as a defaulter so as to levy interest under section 234B of the Act. Thus, even though addition may be justified on account of a retrospective/ clarificatory amendment, interest under section 234B for default of short-payment of advance tax could not be levied.