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
* <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.