Average calendar day at AA means we'll get 5:15 x number of days per trip. You could therefore have a 7 hour day on day one, 2 hour day on day two, 2 hr on day three, and another 7 hour day on day four and get paid 21 hrs. The trip just has to average 5:15/day.
With min calendar day, the minimum per day is 5 hrs and 15 min. So in the above example, you'd get paid 7 hrs on day 1, 5:15 days 2 and 3, and 7 hrs on day four. Min is much better than average (SWA and FDX have this figured out).
The end result will be longer pairings (more 4 days and 5 days), less commutable trips, and more