This is the situation. A purchase transaction should post on the last day of the statement cycle, but is not included for the months interest calculation until the first day of the following month. The amount of the purchase times the DPR for 1 days interest should post to the account. What if it is determined that they are using the number of days in the prior interest cycle plus one day for the calculation; thereby, charging the customer 30 days too much interest? Is there any tolerance for an interest calculation error of this kind? Some corrections could be as much as $20.00. How far back do you think the lookback should be? Two years or back to the beginning of the error?

Assuming your account agreement spells out the way interest should be calculated for purchase transactions, then calculating in any other manner represents a breach of contract. Seek and follow your attorney's advice about the way to cure a breach of this type.