Agree on 72 hours for UK, but haven’t read that anywhere for EU? (mind sharing where you did?) … it just lets you put flight number and arrival / departure time, and doesn’t spit it back at you when I did mine yesterday!
Did just find this ..
"When should travellers complete the digital passenger locator form in the EU application for dPLFs?
Travelling via cruise ship (Before disembarkation)
Travelling via ferry (Before embarkation)
Travelling via train, bus or car (Before embarkation)
Travelling via airplane (After check-in and before embarkation)"