Interest for deferment of advance tax under section 234C accrues monthly on instalment shortfalls against return tax. Interest under section 234C is payable where advance tax instalments are short or late: non presumptive taxpayers face simple interest at a monthly rate calculated on the percentage shortfall relative to tax shown in the return for instalment dates (15 June, 15 September, 15 December, 15 March), subject to concessional limits for early quarters; presumptive regime taxpayers pay interest on any shortfall by 15 March. Certain under estimation of specified income heads is excluded, and 'tax due on returned income' is net of reliefs, credits and TDS/TCS.