public void SetStartTimeBackward(DueTime startTime)
        {
            if (_productionOrderOperation.IsReadOnly)
            {
                throw new MrpRunException("A readOnly entity cannot be changed anymore.");
            }

            DueTime transitionTime =
                new DueTime(TransitionTimer.CalculateTransitionTime(GetDuration()));
            // startBackwards
            DueTime startTimeOfOperation = startTime.Plus(transitionTime);

            _productionOrderOperation.StartBackward = startTimeOfOperation.GetValue();
            // endBackwards
            _productionOrderOperation.EndBackward =
                startTimeOfOperation.GetValue() + GetDuration().GetValue();
        }