Flight date is what matters.

They will always count for the current membership year in which the flight was flown, even if posted late. They will not under any circumstance be counted by the system for a differnet membership year.

Now:
If they post before the 9th (the first day of your next year) no problem. Will get counted for their correct year calculations.

Similarly if they post after the 8th (i.e. 9th or later, after the end of your current year) BUT before the system has processed your year end (which might happen on the 10th etc), then the system should correctly assess you at the year end counting these TP for the previous year, and the TP will be removed along with all other TP from flights flown in the perious year, as a single debit of TP entry.

If they post after the system has conducted your year end process, then the TP will be instantly reversed off after they have posted - because they belong to the previous membership year.
And if they would have resulted in you reaching the required level to attain/retain a status level you will have to get BA to manually look at your account and fix this, late posting TP won't change anything automatically once the year end has happened - it will require manual intervention.

Are you wanting/needing them to attain a new status or just retain a status level?