Interest on shortfall in advance tax instalments arises when cumulative payments miss prescribed instalment thresholds. Interest under Section 234C applies when advance tax instalments are below prescribed cumulative thresholds tied to sequential due dates: the June instalment must meet a minimum proportion, the September instalment a higher cumulative proportion, the December instalment a still higher cumulative proportion, and the March instalment must equal the full assessed tax; interest is charged to the extent payments fall short by each due date.