I highly suggest not modifying core files. The expiration date display is a language string which you can adjust as needed without modifying core files. You can find CBSubs language strings in the below location.
components/com_comprofiler/plugin/language/cbpaidsubscriptions-en-gb/
The language string you're looking for is possibly the following.
Code:
"Active, expiring on %s" => "Active, expiring on %s",