Date of purchase???

DEAR LEARNED  FRIENDS,

Kindly clarify my following doubts.

1. What should be the date of purchase entry when we are importing goods?

a. bill of entry date.

b. date on invoice- (which mostly 45 days earlier)

c. date of arrival at our premises. (normally 10 days after bill of entry).

2. What should be exchange rate for recording the purchase? (will also affect ex-loss/gain calculation)

a. Ex-rate in the bill of entry (which is usually same for the particular period of time)

b. Ex- rate on the date of invoice.

c. Ex- rate on the date of arrival of goods at our premises.

Thanks in advance.