code: public LocalDateTime getOldDateTimeStrict() {
          // If there is no change event, then the old value is the same as the current value.
          LocalDate oldDateValue = datePicker.getDate();
          LocalTime oldTimeValue = timePicker.getTime();
          // If a change event exists, then the old value can be retrieved from the change event.
          oldDateValue = (dateChangeEvent != null) ? dateChangeEvent.getOldDate() : oldDateValue;
          oldTimeValue = (timeChangeEvent != null) ? timeChangeEvent.getOldTime() : oldTimeValue;
          // If either will value is null, then this function will return null.
          if (oldDateValue == null || oldTimeValue == null) {
              return null;
          }
          // Return the combination of the two values.
          return LocalDateTime.of(oldDateValue, oldTimeValue);
      }
docs: getOldDateTimeStrict, This returns the old LocalDateTime value from the DateTimePicker, as it
  would have been reported by getSource().getDateTimeStrict(), before this change event
  occurred.
  
  For additional details, see DateTimePicker.getDateTimeStrict().