PMP Session Downtime Counter

We would like to see a timer, similar to the Session Uptime, in an access point, that shows how long an SM has been offline - the time since it was last registered.  This would help in troubleshooting and determining how long a radio has been offline.  Obviously this won't help if a radio has never registered since the last AP reboot, but for those that go offline, it would be nice to know when they went offline.