Interest computation rules require monthly rounding of periods and rounding of tax amounts for accurate calculation. Procedure for calculating interest payable by the assessee or by the Central Government to the assessee requires rounding the relevant period according to the basis of computation. Where interest is computed on an annual basis, the period must be rounded off to whole months and any fraction of a month ignored. Where interest is computed for every month or part of a month, any fraction of a month is treated as a full month.