Sir,
As per the provisions of the Income Tax Act, the last date for filing of income tax return for a particular Assessment Year is one year from the end of the relevant assessment year. Which means, as on date, you can file return of income for AY 2015-16 (FY 2014-15) and AY 2016-17 (FY 2015-16).
If you are filing return online, the system will accept returns for the above mentioned two assessment years.
If you are in receipt of a notice under section 148, return can be filed online for AY prior to AY 2015-16. In such case, the return shall be mandatorily taken for scrutiny assessment by the department.
Even if no notice under section 148 is being issued by the depatment, return of income can be filed manually before the Jurisdictional Assessing Officer for AY prior to AY 2015-16, bu the return will be considered as defective.