Reading from Supreme Court decision in 365ITR389 it appears material date of purchase or date of transfer of immovable property is the date of agreement to sell as in this case the Court held that a right in personam had been created in favour of the vendee, in whose favour the agreement to sell had been executed after payment of earnest money.
Date of booking or date allotment or date or possession or date of occupancy or date of sale deed etc. are no reference dates to ascertain whether it is long term or short term or for that matter to match the three deadlines for section 54 claim exemption.