Originally Posted by
Ldnn1
It's based on the departure date of the flight, local time of departure.
So if you depart on the 8th it credits to the old year. Depart on the 9th, new year.
Arrival date/time is irrelevant.
And in case anyone asks the usual follow-on question about a flight delayed beyond midnight: it's the scheduled local date and time of departure that counts, not the actual date and time.