My club charges block time as determined by what the hirer tells the invoice writer it was.
(In the very rare cases where the hirer doesn't have a clue because they didn't note the times, calling ATC will get take-off and landing times to which 10 minutes is added.)
Seems to be that all the complications to do with the various inaccurate metering systems are purely there because the business doesn't trust its customers not to routinely defraud them? Makes for a great relationship, no?