Did u mean to say the assessee paid the Advance Tax cheque before 15.03.12. If yes, then the Date of Deposit of Cheque / Presentation of Cheque is the Date of Payment of Advance Tax [Decided in CIT v Ogale Glass Works Ltd.(SC), K. Venkata Reddy v CIT (AP High Court)], . You can write a Letter to the AO stating the above mnetioned facts and also enclosing the copy of the Challan bearing the Date of Payment before 15.03.12.
But if the Advance Tax cheque is deposited after the due date ie. 15.03.12, then the Interest u/s 234C is liable.
Regards,
Devendra Kulkarni