If you are looking for the times that the actual appointment occured then this data is found on the encounter that results from the appointment (including the times at each of the statuses) If you are looking for the planned times, these are in the appointment. An additional comment will be added to the provide examples of where the duration is different, e.g. Where a 15 break is required for a long appointment, and also where an 30 minute appointment is being requested (planned) and no times provided, there could also be other cases. We would not expect an invariant to be defined to restrict this.