If any person, having furnished a return under sub-section (1) of Section 139, or in pursuance of a notice issued under sub-section (1) of section 142, discovers any omission or any wrong statement therein, he may furnish a revised return at any time before the expiry of one year from the end of the relevant assessment year or before the completion of the assessment, whichever is earlier :
Provided that where the return relates to the previous year relevant to the assessment year commencing on the 1st day of April, 1988, or any earlier assessment year, the reference to one year aforesaid shall be construed as a reference to two years from the end of the relevant assessment year.]