Deduction under section 54

Is deduction under section 54 available to an assessee if the assessee owns 2 apartments and applies the LTCG from sale of one apartment to purchase a new apartment within 2 years of sale? Please also advise when does 3 year period for computation of LT asset begin? Is it from the date of the initial allotment of an apartment? Or is it after the full purchase price (over the 3 year construction period) has been paid and the final handover has been completed by the developer?