I would think a cancellation would likely cause one to need to get retested. I don't think a delay of a few hours is a big deal.
The flights depart around 8pm. I don't know how late COVID testing sites are open, but I'll assume you get your sample taken prior to 8pm. So you can't really get a test exactly 72hrs prior. If you get tested first thing in the morning (ie 8am), the earliest you might be able to get a sample taken might be 60hrs prior to departure.
Ofcourse many places say 2-3hrs for a PCR test result so that adds risk that you wont get your result in time.
This is assuming they go down to the minute vs just looking at dates.