Hi,
if you invested the capital gain amount following all the conditions attached to it. in under construction property,
Then period of holding the property starts from the date of allotment of the flat.
AS the word 'property' is used in a wide sense and its definition has re-emphasised this by the use of the words "of any kind". Accordingly, a right to obtain conveyance of immovable property is clearly "property". Therefore, if the booking agreement and allotment terms and conditions of the builder gave a right to obtain conveyance on the said flat, the property after fulfilling certain conditions, that itself becomes "an asset" under the Income-tax Act.
To qualify the investment in case of builder flats, the crucial date is the date of allotment of the residential flat and the payment of installment is a follow up action. Allotment is a sufficient compliance for getting the benefits, even if the taxpayer has not paid all installments due under the said scheme. The provisions of the section are a beneficial provision for promoting the construction of residential house. Thus, the date of issue of an allotment letter gives a right to the taxpayer (Intended buyer) to obtain conveyance on the said flat so that it becomes an asset within the purview of the Income-tax Act. The date of acquisition of the said flat shall be the date on which the allotment letter is issued to the intended buyer.
So, the date of allotment should be considered for computation of period of holding an asset.
Here is a link for your reference to case law similar to your case:
http://taxguru.in/income-tax/date-allotment-date-possession-compute-capital-gain.html