Credit Card Charges & Exchange Rates: Uses trans date or post date?

When using an american card to buy airfare from canadian sites, (airfare usually has a much later post date), does a credit card use the exchange rate on the day the transaction is requested or when it actually posts? The reason I am asking this is I am doing cost comparisons for business and personal travel.|||Call the 800 number on the back of your specific credit card and ask.