Documentation of the sun.rmi.transport.GC.maxObjectInspectionAge() method which calls millis_since_last_gc:

    /**
     * Returns the maximum <em>object-inspection age</em>, which is the number
     * of real-time milliseconds that have elapsed since the
     * least-recently-inspected heap object was last inspected by the garbage
     * collector.
     *
     * <p> For simple stop-the-world collectors this value is just the time
     * since the most recent collection.  For generational collectors it is the
     * time since the oldest generation was most recently collected.  Other
     * collectors are free to return a pessimistic estimate of the elapsed
     * time, or simply the time since the last full collection was performed.
     *
     * <p> Note that in the presence of reference objects, a given object that
     * is no longer strongly reachable may have to be inspected multiple times
     * before it can be reclaimed.
     */

Which means that the way G1 updated this has been wrong as we currently update it for young gcs only.