code: public final ReadOnlyObjectProperty<LocalDate> startDateProperty() {
          if (startDate == null) {
              startDate = new ReadOnlyObjectWrapper<>(this, "startDate", getInterval().getStartDate()); //$NON-NLS-1$
          }
          return startDate.getReadOnlyProperty();
      }
docs: A read-only property used for retrieving the start date of the entry. The
  property gets updated whenever the start date inside the entry interval
  changes (see {@link #intervalProperty()}).
  
  @return the start date of the entry