Just saw this on the CDC website - not sure if it was recently added, or if I missed it earlier:

Why does the Order specify 3 days rather than 72 hours? What is considered 3 days?
The 3-day period is the 3 days before the flight’s departure. The Order uses a 3-day timeframe instead of 72 hours to provide more flexibility to the traveler. By using a 3-day window, test validity does not depend on the time of the flight or the time of day that the test was administered.

For example, if a passenger’s flight is at 1pm on a Friday, the passenger could board with a negative test that was taken any time on the prior Tuesday or after.


Also relevant:

What is a verifiable test result?
A verifiable test result must be in the form of written documentation (paper or electronic copy) of a laboratory test result. Testing must be performed using a viral test (NAAT or antigen), and negative results must be presented to the airline prior to boarding. The test result documentation must include information that identifies the person, a specimen collection date and the type of test. A negative test result must show test was done within the 3 days before the flight. A positive test result must show the test was done within the 3 months before the flight.