Due date of income tax return for the f.y. 2015-16

F.Y 2015-16, Ass Yr. 2016-17. Last date of filling belated return u/s. 139(4), is 31.03.2018. 

If the person fails to file the return of income within the time-limit prescribed in this regard, then as per section 139(4) he can file a belated return. A belated return can be filed within one year from the end of the relevant assessment year or before completion of assessment, whichever is earlier.

Refer : https://blog.makemyreturns.com/post/efiling-of-income-tax/section-139(4)-belated-return.aspx

Your confusion may be due to amendment under the section as per finance bill 2016, effective from AY 2017-18

F.Y 2016-17, Ass Yr. 2017-18. Last date of filling belated return u/s. 139(4), is 31.03.2018.