We explicitly chose to be the instant data type for force the type. We didn't identify any use cases where you get an appointment with only a date(s), and no time component. Cases where the exact time isn't known, but have a range of times such as home care where the appointment might be between 9 and 10 for 15 minutes is also supported with the current structure. The other cases we had were where there was a request to schedule an 30 minute appointment sometime next week, which is covered through the requestedPeriod property.  Logically when you get an appointment, you are always provided a time, otherwise its a requestedPeriod. Scheduling systems which manage appointments all work on dates and times.