OK here is my situation.

Fully vaccinated, US passport, CDC card. Have SFO-FRA-BCN ticket, departing SFO at 15:00 local time or almost midnight in France.

The plan is to stay one night or two in Barcelona and then fly to NCE.

If I spend one night in Barcelona, I could arrive in Nice before 18:00. So if I take an antigen test in SFO after 12 PM or 21:00 in France on 21 June, I would be arriving in NCE about 3 hours before the end of the 48-hours period on 23 June, which would be 21:00 that day.

Will it work? I won't need a test result in FRA or BCN according to the latest rules. But I could reach NCE before the 48-hours have expired.

Or bring along a home Antigen test for entry in NCE? I would be taking a Vueling flight for BCN-NCE.