Can i file return of ay 15-16 back dated online?

You can file IT return for Ass Yr. 2015-16, under section 139(4), online or manually, till march 2017, provided not filed earlier.

But can i file it back dated i.e. if i file the return of AY 15-16 today, can i put the date as 10th april 2016 and the acknowledgement will show the same date? please reply..

First tell me why you want to file back dated? What specific idea in it?

No, it will shown you the current date. Also there is no provision to file return back dated.

Client wants to take a working capital loan and bank policy is that there should be a gap of minimum 6 months between two returns i.e. AY 16-17 & AY 15-16

for the AY 2015-16 until march 2017 the returns can be filed online. The return acknowledgment will show the date on which the same has been actually filed.

howver if the return is under tax audit - you can request your auditor to give a back date on the audit report( the date of auditors report will be of back date) 

even if the return is filed offline - by submitting it to the IT office - the date of filing the return will be marked in the acknowlegement.