Pinning this to the top for a while, if people think it's helpful.

I will say that how I have dealt with it is as follows, kind of backwards from Linda's thinking..

OK, my Covid test is 11 AM on Monday. It is a PCR test, so I have 72 hours before my flight to have my test. So, 24 hours from 11 AM Monday is 11 AM on Tuesday. 48 hours is 11 AM on Wednesday. 72 hours is 11 AM on Thursday. SO--if I take my test at 11 AM on Monday, and everything goes off on time, my flight to SXM needs to take off before 11 AM on Thursday.