Purchase date and value in case of import (trading)

We import prodcuts for trading purpose on CNF basis. Supplier raises Invocie in USD the date on which Bill Of lading is provided by shipping line.

Our payments are 20% advance agaisnt PI and Balance on reciept of documents.

Question:

What date should be taken as the pruchase date ?

1. Date of Invocie
2. Date when I receive the Copy/Original Invoice
3. Date on which final / balance 80% payment is made
4. Date when vessel reaches Port 
5. Date of Bill of Entry is passed by customs 

Which exchange rate to use for the conversion ?

1. Actual rate at time of payments
2. RBI referecne rate as on date of purchase
3. Customs reference rate on date of pruchase

In case the USD rate from Q2 is 2 or 3, then how / under which ledger to account for the difference in purchase cost and actual amount remitted.