// In XAML:   

       <gantt:GanttControl x:Name="ganttControl" Grid.Column="1" IDBinding="{Binding TaskID, Mode=TwoWay}"

                           EffortBinding="{Binding EndTime, Converter={StaticResource effortconverter}}"                                       EndTimeBinding="{Binding EndTime, Mode=TwoWay}"

                       ....................      

                           >           

        </gantt:GanttControl>

    public class EffortConverter : IValueConverter

    {

       CustomTask task;

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)

        {

            task = (CustomTask)parameter;

            return GetTaskEffort(task);

        }

        private object GetTaskEffort(CustomTask task)

        {

            if (task.StartTime != null && task.EndTime != null && task.StartTime != DateTime.MinValue && task.EndTime != DateTime.MinValue)

            {

                // Note that "WorkTimeSchedule.Schedule8X5" is the same schdule used by the GanttControl

                // in this app. If you use a different schedule in the GanttControl, use that same schedule here as well:

                return RadiantQ.Windows.Controls.Gantt.WorkTimeSchedule.Schedule8X5.GetEffort(task.StartTime, task.EndTime);

            }

            return TimeSpan.Zero;

        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)

        {

            throw new Exception("Not implemented.");

        }

    }