Return a const reference to the latest date in the valid range of this timetable. The behavior is undefined if this timetable does not have a valid range (i.e., it is in the default constructed (empty) state).
Return a const reference to the latest date in the valid range of this timetable. The behavior is undefined if this timetable does not have a valid range (i.e., it is in the default constructed (empty) state).