Excellent corner case. Here it comes down to how you model the storage of your appointments. Since the issue is framed within a calendar the "instant" itself is

Excellent corner case. Here it comes down to how you model the storage of your appointments. Since the issue is framed within a calendar the "instant" itself is not of primary concern, but the position within that calendar. This is also how the likes of "chron" model scheduling of tasks. I found this so hard to wrap my head around at first precisely because I was used to dealing with time "as a programmer" from the inside out. Of course when you're scheduling a repeating task that you want to happen "at midnight" or "on a Monday morning" it makes more sense to specify in this top-down manner. I might categorise such things as "a presentation layer" concern, but as you point out, in these cases your moment in time is stored as a calendar reference.