Thank you ericferret, but I do know what time in service means, and it's not relevant here.
I'm looking for the definition of "Hours of operation".
It isn't nearly as rigid as you think. It is down to the agreed maintenance schedule. Cessna do not have a 'must be this way or else your plane will fall out the sky' version of operating hours.
My authority, the French DGAC via its contractor OSAC
is definitely rigid. The long time custom in France is to construe "hours of operations" as "Block time". And I do need OSAC's approval of my maintenance programme if I want to use my aircraft for flight lessons.
I got an Email from Cessna telling me that in their maintenance manual, "Hours in service" means the value given by the G1000.
That value is usually between airborne time and block time.
Although I've not found any Cessna document to substantiate my surmise, I think that the total hours of operation value given by the G1000 is 1/60 of the ratio of the number of revolutions of the crankshaft over a given RPM value