WPF Date picker selected date does not get reset after deleting date

I  have extended WPF default datepicker control and created a custom date picker with masking in the date picker text box.

For this I overridded the template for datepicker text box like below:

<Style x:Key="DatePickerTextBoxStyle" TargetType="{x:Type DatePickerTextBox}"><Setter Property="Height" Value="20" /><Setter Property="Control.Template"><Setter.Value><ControlTemplate><TextBox x:Name="PART_TextBox"
                             Style="{DynamicResource CalendarTextBoxStyle}"
                             TabIndex="0"
                             Text="{Binding Path=SelectedDate,
                                            Mode=TwoWay,
                                            StringFormat='d',
                                            ConverterCulture={x:Static glob:CultureInfo.CurrentCulture},
                                            RelativeSource={RelativeSource AncestorType={x:Type maskedDatePickerLib:MaskedDatePicker}}}"
                             TextWrapping="Wrap"><i:Interaction.Behaviors><maskedDatePickerLib:TextBoxInputMaskBehavior InputMask="00/00/0000" PromptChar="_" /></i:Interaction.Behaviors></TextBox></ControlTemplate></Setter.Value></Setter></Style>

I have created a interactivity behavior (for masking) and attached to a text box and put it in a date picker text box template. 

Now the problem is when I select a date from date picker calender and delete it from text box manually. Then if I again try to select the same date it does not appear in textbox.

When I used Snoop in the application I saw that even after deleting the text from text box. SelectedDate property of date picker still have the date. Kindly suggest.