ITR2- Long Term Capital Gain of property purchase in installment due to under construction

I am confused when tried to fill Land or Building section of “Schedule Capital Gains” in ITR2. Online system is giving option only to fill “date of purchase/acquisition” and “Date of Sell/Transfer”. But what would be the “date of purchase/acquisition” when I brought flat in installment because it was under construction when i signed agreement with builder/developer ?

Here is my scenario:

I booked and signed on Sell Agreement with builder/developer on Dec 2010. I paid 1 Lakh during agreement sign with an alignment that my bank or I need to pay 10% with every floor roof casting . There were total 10 payments till Dec 2014 when all my dues to developer were paid by my or from my bank and flat was ready to move. They gave me possession letter on Dec 2014 and registered the flat on our name same time. During registration we spent all stamp duties and legal fees. Finally I sold this flat unit on last July 2022. 

Please help to clarify, how to calculate Capital Gains in this case when in ITR2 only “date of purchase/acquisition” and “Date of Sell/Transfer” option available. Which date is correct for “date of purchase/acquisition” - Dec 2010 or Dec 2014 ? How to consider 10 prorata basis payments (2010 to 2014) in LTCG calculation?    Thanks in Advance !