You might find an answer in the thread on Rule Machine Time Variables. In post 1, Bruce says that you can calculate the difference between two time variables, so you should be able to do exactly what you describe. Look into Global Connector as a way to get this variable into something you can display on a dashboard.
If for some reason that doesn't work out, you can come back to my alternate idea of letting the Zooz keep time for you by resetting energyTime upon each event, and therefore you can use energyDuration as the time between the stops.
