Capital gains - joint development agreement - date of acquis

we had entered into a joint development agreement on sept 2010, whereby for the land surrendered 2 flats were alloted.  the flats were constructed and handed over during Mar 2012.  Further the flats were sold during Oct 2015. 

For indexation of cost of acquisition pls clarify whether 2010-11 has to be taken or 2011-12 has to be taken.

Thanks

n t vinodh