Customer says we have been charging the incorrect interest rate on the credit card account since 1997. (Whether or not this is true, and it probably is, depends on customer's other relationships with our bank - if total relationships exceed a certain level, customer receives a variable rate, if not, the rate is fixed and higher.) Does this fall under the billing error provisions of Reg Z (226.13) and require that we be notified within 60 days of the statement, or is this a contract issue, and are we required to reimburse all excess interest charged since 1997? The only description of a "billing error" that I see could fit is 226.13(5): "a computational or similar error of an accounting nature that is made by the creditor." Even if it is a billing error and subject to those rules we will in all likelihood reimburse most if not all of the interest, but I would like to know if we have any discretion as I think the calculation of the amount may not be such a simple task. Thanks!

Sounds like a contractual issue to me.

I agree with Randy - it is a contract issue. That means that the statue of limitations will be based on requirements of state law.

Before you start re-calculating finance charges for 6 years, you might want to consult with counsel. You might be able to get out of refunding on some or all of the differences.