I have seen in credit card and even debit card transaction two dates associated with each completed transaction. The date it was processed by the bank which one of my cards calls the post date, and the date the card was scanned by the merchant which they call the transaction date.
Many times the dates are the same, many times they can be separated by a day or even two. I have seen this with online transactions, in person transactions, and even ones were there is a two step process such as at a gas station or restaurant.
Unless the data you are working with has both dates, then you have to hope the OFX file date matches the one in the monthly statement. In the one card I was looking at today it is the transaction date that is the main one they show on the monthly statement.
