I called Seabourn today. The policy is anytime 3-days or less, not 72 hours from boarding time. They previously had a 72 hours requirement but it was too cumbersome for guests to calculate out the time changes, specific boarding times so they dropped that. So for now, as example, I verified that if you had a sailing date of July 17th, testing ANYTIME on the 14th or later will suffice.