For Income Tax purposes the company will have to close its accounts on 31.03.2013 and Income Tax return shall be filed for the A.Y. 2013-14. .
(As per Section 3, in the case of a business newly set up, P.Y. shall be the period beginning with the date of setting up of the business and ending with the said financial year i.e. 31st of March)
Every company shall, on or before the due date, furnish a return of his income during the previous year
Hence, your client co. is required to file two seperate returns of income.
The option of 15 months is for holding Annual General Meeting(under companies act,1956) & NOT for filing of return of income.