How far a is AHEAD of b at at, in minutes — positive when a's
clock reads later (Zurich vs Los Angeles in June → 540), negative when
earlier, 0 when they agree at that instant (two zones can differ in
general and still agree today: London and Lisbon, or Phoenix and Denver in
winter). Compare at the appointment's instant, not now: an offset can
change between booking and the visit.
How far
ais AHEAD ofbatat, in minutes — positive whena's clock reads later (Zurich vs Los Angeles in June → 540), negative when earlier, 0 when they agree at that instant (two zones can differ in general and still agree today: London and Lisbon, or Phoenix and Denver in winter). Compare at the appointment's instant, not now: an offset can change between booking and the visit.