Capital gain exemption on new purchase of under construction property

in case of purchase of a residential flat which is under construction,how is the time limit of purchase or construction applied?

old flat is not sold yet and booking of new flat is done and part payment is done and agreement to sell will be done next week.the possession will be given after 2 to 3 years.

so what should be the date of sale of an old house if planning to sell in the coming month so as to be eligible for claiming exemption.I mean if this new purchase is considered as construction then whether agreement made before one year from the sell date will be eligible or 1 yrs before limit won't be available.