I booked an under-construction flat in Mumbai in October 2024, and got the agreement registered a month later. The last payment to the builder is due in December 2026. I also own a 10-year-old flat which I intend to sell to fund the last payment. In order to claim long-term capital gains (LTCG) exemption under Section 54 of the Income-tax Act, 1961, by when do I need to sell my flat? Is the date relevant for Section 54 the registration of agreement or the last payment for the under-construction flat?