In the case of any person, who in the Income tax officer 's opinion, is assessable under this Act, whether on his own total income or on the total income of any other person during the previous year, the Income tax officer may, before the end of the relevant assessment year, serve a notice upon him requiring him to furnish, within thirty days from the date of service of the notice, a return of his income or the income of such other person during the previous year, in the prescribed form and verified in the prescribed manner setting forthwith such other particulars as may be prescribed; Provided that on an application in the prescribed manner the Income tax officer may, in his discretion, extend the date for the furnishing of the return, and when the date for furnishing the return, whether fixed originally or on extension, falls beyond the 30th day of September or, as the case may be, the 31st day of December of the assessment year, the provisions of sub clause (iii) of the proviso to sub section ( 1) shall apply.