Import purchases

exchage rate = bill of entry custom exchange rate

date of booking may be bill of entry date / MRN date/inv date  as the case may be

it all depends on your purchase terms(Wheher it is ex-works, or FOB, CIF)

what ever it is 

transaction of import should be recognised when all risks and rewards of ownership transfered to company  

and the difference between the exchange rate as at the date of the transaction and as at the date of settlement thereof should be recognised as income or expense,  depending on the nature of the item to which the exchange difference(s) pertain, as per the requirements of AS 11,

Note:

if purchases is asset it should be adjusted to cost of asset as per AS11 and should not treat fluctuation as income or expence

In general RBI exchange rates will be used for forex fluctuation entries