As per section 139(4) of Income Tax Act 1961 Belated return can be filled within end of 1 year from end of the relevant assessment year. For FY 15-16 the AY is 16-17 ( End of AY is 31st March '2017) .So we get further time of 1 year from the end of relevant AY i.e we can file the return by 31.03.18
For your information from AY 17-18 onwards this time limit is reduced by 1 year and return for the AY 17-18 has to be filed by 31.03.18