Flights taken on the 8th of the month (as T8191 notes, all TP collection years end on a 8th) credit to the current year, ending on that day.

For the purposes of being on the 8th, it is scheduled local departure time that counts, so a flight departing the West Coast of USA in the evening of the 8th will still credit to the existing year, even if it is already the 9th by local time at Heathrow.

Also as noted, this is applied to individual sectors, so if you take off from e.g. SFO on the 8th, and land on the 9th then connect up to GLA, then the long haul gets credited to the previous year, the short haul to the new year.