This just in:
"The Court concludes that the 'arrival time', which is used to determine the length of the delay to which passengers on a flight have been subject, corresponds to the time at which at least one of the doors of the aircraft is opened, the assumption being that, at that moment, the passengers are permitted to leave the aircraft," it said.
http://uk.reuters.com/article/2014/09/04/uk-eu-airlines-ruling-idUKKBN0GZ0VE20140904