Originally Posted by Andrew Kim

48 hours before the SFO-NRT flight; the start of your journey. And they want you to think in terms of days (2), not hours, which allows for a more liberal interpretation of 48 hours that benefits the traveler.

So say a first flight is scheduled to depart at 23:59 on a Wednesday. The traveler can get their test as early as 00:00 Monday (bad example as no clinics will be open at that time, but my point stands) and still have those 71+ hours count as "2 days ago."

How much time passes while you were en route to Korea is irrelevant as long as you continually remain a transit passenger and do not enter any country where you make connections.

I think this system makes things equal for both the flyer who lives in LA and gets to take a direct flight to Seoul, and another who might have to go from Portland to Seattle to Narita to Seoul.