What counts as a day? I imagine that a half day in two countries would be counted as one day towards both since neither is probably splitting hairs over whether it was 12 hours and one minute. I would imagine that because of this it is possible to "double spend" a day for every time you fly between the same two counties.

Obviously this assumes that given a range of dates, the number of days is calculated as inclusive of the date of ingress and egress.