Calculation of the 3 year period for capital gains tax

I’m planning to sell my flat and reinvest the money in buying another flat. How should I proceed to save any short term capital gains tax if applicable?

Issue 1:

I had booked a flat in July 2009. In order to compute the period for calculation for the 3 year window for Capital gains tax, which date is important of the below:

1.       Date of Booking of Flat: July 2009

2.       Date of Allotment of Flat: Nov 2009

3.       Date of Builder Buyer Agreement: Mar 2010

4.       Date of Possession: Still waiting for it

Issue 2:

Does payments play a role is calculation of the 3 Year window period?

I had made around 20% payment when the Allotment letter was issued and had made a payment of around 35% when the builder buyer agreement was executed. By now I have paid around 90% of the money to the builder.