Flight time
At least in the US, Airworthiness limitations are always clearly specified as to what they are based on be it flight-time, run-time (which I have never seen as a metric for a life limit), landings, cycles / cycle computations or calendar time. The Hobbs type meter is not required in any helicopter unless it is called out in the type certificate. You are always legally allowed to use your giant pilot wrist watch, stop watch, cell phone timer, or an egg timer to log flight time. The meter is a convenience only be it engine/transmission oil pressure activated or a collective activated switch. Of course many late model ships have timers and cycle counters as part of a VEMD or other built in data recorder. Most operators use flight time for maintenance actions and run time for billing customers but that varies.