I am so sorry that I was not clear. Your original question was if the 3 days could be also be seen as 72 hours. Here is your quote:
"The language for getting a covid test 3 days prior to your cruise could also been seen as 72 hours, correct? There are several different opinions of this andf I say it's 72 hours prior. What say you?"
Therefore, my answer was that no, that is incorrect. The 3 days and 72 hours are very different in the covid testing world.
And jeeez... of course technically 3 full days are 72 hours. In the last year, cruises were quite clear about testing and selected to use the number of days versus the number of hours.
Now I will answer your question about when the 3 days begin. It was quite the topic of conversation last year. I only know that because we were on two cruises last year.
If your ship departs on a Sunday, you can test beginning on Thursday. You can test beginning at 12:01 am on Thursday which will technically be MORE than 72 hours by the time you are actually boarding your ship on Sunday. And this is why the number of days has more comfort space in it.
So, if you board on the 17th, you may test as early as 12:01 am on the 14th.
I hope this helps.