This read-only TimeInterval object reports how many seconds remain before the oldest currently active TEK for this FSM expires, or zero if there are no active keys at all. Because BPI maintains overlapping old and new keys during a key transition, this object lets an admin see how close the older key is to retirement. For example, if cwrBpiRsTEKExpiresOld reads zero on a radio that should still be actively encrypting, the admin would recognize that key has already lapsed and investigate the rekey process.