Interest for default in filing ROI accrues monthly until the return is filed or assessment proceedings conclude. Interest for default in furnishing the return of income is a simple monthly charge from the day after the due date until filing or completion of assessment. The taxable base is the tax determined on assessment mechanisms, reduced by tax deducted/collected at source, advance tax, specified reliefs and allowable tax credits; self assessment tax paid on or before the due date is deductible. Interest already paid reduces liability and interest is adjusted where subsequent orders alter the tax base. For self assessment tax, the tax declared in the return substitutes as the tax base for interest computation.