Advance tax deferment interest: insufficient instalment payments attract simple interest on the shortfall, with limited statutory exceptions. Interest is payable where an assessee liable for advance tax pays less than prescribed proportions by instalment dates or less than the tax shown in the return at the final instalment; such shortfall attracts simple interest on the deficiency, subject to limited reliefs and exclusions for certain under estimated incomes and where attributable tax is paid by the terminal date. The base for calculation is the tax due on the returned income, reduced by specified deductions, reliefs and tax credits.