At some airlines, I believe, it is the time that the dispatcher hands over the load sheet, and the time that the Captain rights on it, as that is the time at which the aircraft is considered to be ready to go, barring any ATC resritiction. Personally, I write down the time that we are given permission to push, after that it is a matter of fate, if you like. I don't think we have it defined in our Ops Manual.