Advance tax shortfall interest applies on unpaid advance tax, attracting calculated interest until regular assessment is complete. Where advance tax paid on the assessee's estimate is less than the prescribed proportion of assessed tax, simple interest at the prescribed annual rate is chargeable from the 1st April following the financial year until the date of regular assessment on the shortfall; payments made before completion of assessment adjust the interest calculation, reassessment or revision increasing or reducing assessed tax adjusts interest correspondingly, and the Assessing Officer may reduce or waive interest in prescribed cases.