Indexation consideration

Hello Everyone out there,

I have a question. Actually my client had purchased property. conveyance deed is dated 15.12.2018, Agreement to sell is dated 24.04.2008. Allotment letter dated 29.07.2009 and possession letter is 21.12.2016. Payment for purchase of property started from 24.04.2008. We are confused now. What should be the indexation year. Whether we should take indexation year? From payment date or possesion date or agreement date? Please help.