Thanks for your reply,
first, you are right about warning us about time zone issues. We are very concern about this and we do notice that Appointment and Slot "start" and "end" use "instant" datatype. By the way I believe that there is sometimes a mix behind the meaning of time zone and time shift. 'instant" datatype does handle time shift which is enough but does not handle time zone. The difference is that time zone can have different time shift depending on season (daylight saving) or local political choices at key dates. More over multiple time zones have the same time shift, thus you cannot know in which time zone you are based on time shift.
About my question, we are currently working on recurring Slot, not Appointment. The goal is to exchange that information between two systems that will handle by their own the availability computation. Each system will “expand” the slots into something real. We do need this because we cannot afford having one system requesting the other to get Slots for performance reasons.
I hope this help