You can't have a student logging a different time to the instructor, nor can it be at variance to the Act.
I don't think anyone was saying in variance to the act. I log my time from the GPS logs so the discrepancy between myself and the student would be the taxi time rarely more than a minute or two and certainly absorbed in the 'swedish rounding' used to take the numbers to the nearest five minutes.
Almost every R22 I've known clocks datcon time on engine oil pressure, ie from engine start to engine stop
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My experience of the type is rather more limited but I was assured that it was from the collective.