Hi Dipak.
Happy Dussehra!
the date on which the flat is allotted to you or the agreement is signed between builder and you where conveyance title of the flat is passed to you, would be considered as the date of acquisition of the flat.
if the date of allotment of the flat is aug 2011, then same would be considered as date of acquisition of flat and would be indexed to arrive at capital gain.
it will come under as long term capital,
here is the case law for your reference and understanding.
http://taxguru.in/income-tax/date-allotment-date-possession-compute-capital-gain.html