Use Calendar instead of MemoryCalendar in interfaces
We don't need any of the MemoryCalendar API.
By using Calendar in the interfaces the API gets more flexible and we can use it for non-MemoryCalendar-based calendars
We don't need any of the MemoryCalendar API.
By using Calendar in the interfaces the API gets more flexible and we can use it for non-MemoryCalendar-based calendars