Which date as acquisition date of land, for capitalgains?

Sir,

Booked a property through haryana  government in 2001. This was through some lottery. Kept paying instalments till 2011. The property was a residential land. For the purposes of calculating  captial gains, which year should be treated as the date of acquisition ? When the property got booked ? Or when the the final amount was paid and we got possession ? 

I have read all about the tax libility for this on the internet and this is the only point I am unsure about. I need to know this to calculate the inflation-adjusted buying amount... also known as indexed amount.

Thanks