code: public final ReadOnlyObjectProperty<ZoneId> zoneIdProperty() {
          if (zoneId == null) {
              zoneId = new ReadOnlyObjectWrapper<>(this, "zoneId", getInterval().getZoneId()); //$NON-NLS-1$
          }
  
          return zoneId.getReadOnlyProperty();
      }
docs: A property used to store a time zone for the entry. The time zone is
  needed for properly interpreting the dates and times of the entry.
  
  @return the time zone property