It is not true that income tax return once filed cannot be revised but only return filed after due date can’t be revised. Every person, being a person specified in S. 139(1), is required to furnish a return of his income on or before the due date. The due date for filing income tax return for corporate assessee’s and other assessee’s who are required to get their accounts audited under Income Tax Act 1961 or under any other law for the time being in force is 30th September and for others it is 31st July every year as have been prescribed u/s 139(1).
However if you still miss filing the return, then you can file it by 31st March under section 139(4) and that will be called Belated Return under Income Tax. If there is a tax liability then interest will be charged at 1% per month. The income tax department may impose a penalty of Rs.5000/- for late filing of returns i.e. after 31st March. One cannot file late return after one from the end of assessement year eg. return for FY 2013-14 due date is 31st July 2014 but one can file late return without any penalty upto 31st March 2015 and last date is 31st March 2016 but in that case ITO can impose penalty of Rs 5000/-
Revised Return: If you find any errors or omissions in your return, then you 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. A return can be revised any number of times. However, a belated return cannot be revised.
Problems related to late filing of return
CARRY FORWARD OF CERTAIN LOSSES : if return is not filed on or before due date then one can’t carry forward business or capital losses for set off against future profits and gains for that year. In order to carry forward such losses, it is mandatory to file the return on time. However, the loss from house property & unabsorbed depreciation does not have this limitation and can be carried forward even if the return is filed late. Carry forward of losses (other than loss from house property and unabsorbed depreciation) is permissible if the return of income for the year, in which loss is incurred, is filed in time. The late filing of return should not impact the status of carry forward of loss of previous years.
REVISION OF LATE RETURN NOT POSSIBLE : A late return cannot be revised, unlike a return filed within the original time limit, which can be revised within a specified period. So this means that once a person has missed the original deadline, he needs to be extra careful, as a late return cannot be revised.
LOSS OF INTEREST ON REFUND: When return is filed after due date then interest on refund is paid only for the period from the month of filing the return to the date of refund. So person with refund should file the return before due date, so that he can get the interest for the period from 1st April of the assessment year.