Silverlight Binding TimePicker

I have a data form with a timer and I cannot change the value in timepicker when I bind the value in XAML. Here is the code for xaml binding:

<toolkit:DataField Label="Start Date:" Width="325">
                            <sdk:DatePicker x:Name="StartDate" SelectedDate="{Binding StartDate, Mode=TwoWay, Converter={StaticResource DateConverter}}"/>
                        </toolkit:DataField>
                        <toolkit:DataField Label="Start Time:" Width="325">
                            <toolkit:TimePicker x:Name="StartTime" PopupButtonMode="Hover" Value="{Binding StartDate, Mode=TwoWay}"/>
                        </toolkit:DataField>
                        <toolkit:DataField Label="End Date:" Width="325">
                            <sdk:DatePicker x:Name="EndDate" SelectedDate="{Binding EndDate, Mode=TwoWay, Converter={StaticResource DateConverter}}"/>
                        </toolkit:DataField>
                        <toolkit:DataField Label="End Time:" Width="325">
                            <toolkit:TimePicker x:Name="EndTime" PopupButtonMode="Hover" Value="{Binding EndDate, Mode=TwoWay}"/>
                        </toolkit:DataField>

What changes can I make so that the user can change the value in timepicker.

Thank.