I don't know the answer but assume that you are wise to assume that if your flight is delayed and your arrival is going to be more than 72 hours after your test was carried out, there's a problem. Would the airline gate agent who is announcing the delay alert people to the fact that they should check the time their covid test was done, and if necessary, head down to the area in the airport where 1 hour rapid tests are being carried out, assuming the airport has such a service? O'Hare does, for a fee of $120. Or will the gate agent be oblivious to this, and people waiting for their delayed flight will have to take it upon themselves to do something like that?
I'm stressing out about these 72 hour timing issues too. I was researching it on an Italian government website, hoping for more guidance on the 72 hour rule. The language I read was that the test must be "carried out" within 72 hours of arrival. What does that mean? Sample drawn no sooner than 72 hours before the flight lands in Italy? Or results dated no sooner than 72 hours before the flight lands? It's two very different things if you go to a testing facility that has a 24 to 48 hour wait for results.
Presumably anyone looking at your test results in Italy will do the time zone conversion. You've already got some extra time built in due to the fact that Italy is 6 hours ahead of EST in the US. If you arrive at, e.g., 9 a.m. on Sunday, Italy time, the count-back doesn't go back to 9 a.m. on Thursday in the US. It would be 3 a.m. Thursday. So long as you carried out your covid test after 3 a.m. Thursday, you'll be okay. But again, does that mean sample drawn, or the time stamp on your results? Does anyone know?