It's the time taxi-ing without engines on, eg under tow or push back

The second is potentially subjective, but I'd count it as the moment the engine is actually running, post-start up procedure. I've the moment when I could use the engine as an actual engine....

It's the time taxiing without engines on, for example when under tow or during push back. As opposed to when you're taxiing under the aircraft's own power

The second is potentially subjective, but I'd personally count it as the moment the engine is actually running, post-start up procedure: that's the moment when I could use the engine as an actual engine with useful power.