Interest on under estimated advance tax requires assessee to pay simple interest for deficient or deferred instalments under assessment. Where, on regular assessment, the Assessing Officer finds that an assessee has under estimated advance tax or wrongly deferred advance tax payment, the officer may direct payment of simple interest for the period of deficiency or deferral. For under estimation, interest is charged on the difference between amounts actually paid in each deficient instalment and the instalments that should have been paid, having regard to aggregate advance tax paid. A deemed due date rule treats instalments due before six months into the previous year as becoming due fifteen days after that six month period.