It's the expiration date of the subscription as CBSubs sees it. That's basically as good as it's going to get from CBSubs perspective.

When a renewal payment is received CBSubs will extend their current subscription by the duration of the plan. If a renewal is not received it'll expire on their expiration date. If the gateway uses scheduled API in CBSubs instead if IPNs and CBSubs fails to make a payment it won't renew and will expire on their expiration date otherwise the renewal will extend it by the duration of the plan.

There is one last possibility I just thought of (with Authorize.net): The only place where the credit-card expiration date (without credit-card number of course) might be stored is in the corresponding payment subscription notification at the first payment. By exploring that table in PhpMyAdmin you might be able to find it. But there you are on your own to find a corresponding SQL query, as the information is probably inside a logging field, without guarantee that the format stays the same in future versions.

However this will never cover the case where the card's credit doesn't allow a payment, or the card is cancelled before expiration.

Authorize.net AIM is our only gateway where that card expiration date information may be available in the history logs. We replace the card number by XXXX's in all logs as required by PCI-DSS standards. Just as a reminder (I'm sure Authorize.net already required that: With Authorize.net AIM method, your server must be PCI-DSS certified.