score:5
To passengers, an airline considers flight time the period from when the cabin door is closed to when the door is opened. Similarly, EU261 considers the time of door opening for compensation. Such data is reported automatically from the aircraft over a company data link and shown on their website.
FlightAware, because its based on ATC and aircraft navigation data, can't see these events, so it isn't accurate in determining this time. For example, if the engines are shutdown, but the door can't be opened because there's no jetway driver, FlightAware would be definitely incorrect.
An alternate means of measuring flight time is the time it lifts off until it lands, which is used for technical purposes (aircraft maintenance). While a valid measure, it isn't relevant for EU261.