As per my knowledge First of all you just check when risk and reward associated with goods is transfered to us and when the liability of the party arises? the day when liability arises.... it should be the transaction date.
say for example goods is dispatched by A from japan to Mr. B in india and invoice for the same is raised dated 10th June 2012 however the the bill of ladding date is 15th june 2012. then you have to see whether risk of the goods is transfered to Mr. B on 10th itself or on 15th ?
if the risk transfered as and when invoice generated then rate prevailing of 10th should be taken and in case two where risk transfered on 15th then rate prevailing on 15th will be taken
Hope it is cleared to you.
Regards
Madhusudan Kabra