A read-only TimeInterval giving the elapsed time since the subscriber unit last successfully synchronized with the head end. A small value confirms the unit is currently in sync and stable, while a large or growing value alongside a healthy sync flag would be unusual and worth double-checking. For example, an admin verifying a link's current stability could poll p2mpSuLastSyncSuccessTime and see it holding steady near zero increments, confirming the unit has remained continuously synchronized rather than repeatedly dropping and reacquiring.