Binding problem in DataTemplate with custom Control - get

Question

I have a DataGrid which binds an ObservableCollection. The DataContext of MainWindow is set to MainWindowViewModel and this contains the ObservableCollection for the DataGrid.
In the DataGrid i have a DataTemplate. When i bind the Collection to a standard control lika a Label, the databinding works fine, but when i used a custom control, the databinding does not work.
My XAML:
<Window.DataContext>
<local:MainWindowViewModel />
</Window.DataContext>
...
<DataGrid ItemsSource="{Binding ShiftTimes, Mode= TwoWay, UpdateSourceTrigger=PropertyChanged ....}">
<DataGrid.Columns>
<DataGridTemplateColumn>
<DataTemplate>
<cControls:MyUserControl SelectedTime="{Binding Path=From}" /> <-- DOES NOT WORK
<Label Content="{Binding Path=From}"/> <-- Works fine
</DataTemplate>
</DataGridTemplateColumn>
</DataGrid.Colums>
</DataGrid>
My control is a XAML-ControlTemplate and the the specific property "SelectedTime" is set by
...
<StackPanel DataContext = "{Binding Path=SelectedTime, RelativeSource={RelativeSource Mode=FindAnchestor, AnchestorType=local:MyUserControl}}>
...
the "SelectedTime"-Property is a DependencyProperty
the code behind my UserControl:
public TimeSpan SelectedTime
{
get { return (TimeSpan)GetValue(SelectedTimeProperty); }
set { SetValue(SelectedTimeProperty, value); }
}
public static readonly DependencyProperty SelectedTimeProperty =
DependencyProperty.Register("SelectedTime",typeof(TimeSpan),
typeof(MyUserControl),
new FrameworkPropertyMetadata(MyUserControl.MinValidTime, new PropertyChangedCallback(MyUserControl.OnSelectedTimeChanged), new CoerceValueCallback(MyUserControl.CoerceSelectedTime)));
the debugger gives the following output:
System.Windows.Data Warning: 67 : BindingExpression (hash=14315403): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=14315403): Found data context element: MyUserControl (hash=1590600) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=14315403): Activate with root item ShiftTime (hash=53855148)
System.Windows.Data Warning: 108 : BindingExpression (hash=14315403): At level 0 - for ShiftTime.From found accessor ReflectPropertyDescriptor(From)
System.Windows.Data Warning: 104 : BindingExpression (hash=14315403): Replace item at level 0 with ShiftTime (hash=53855148), using accessor ReflectPropertyDescriptor(From)
System.Windows.Data Warning: 101 : BindingExpression (hash=14315403): GetValue at level 0 from ShiftTime (hash=53855148) using ReflectPropertyDescriptor(From): TimeSpan (hash=0)
System.Windows.Data Warning: 80 : BindingExpression (hash=14315403): TransferValue - got raw value TimeSpan (hash=0)
System.Windows.Data Warning: 89 : BindingExpression (hash=14315403): TransferValue - using final value TimeSpan (hash=0)
Thanks for Your help
UPDATE:
I found the reason. In my Control-Constructor i set the SelectedTime-Property explicit to a value.
After deleting this, the binding works.
In this thread -> https://stackoverflow.com/a/66698078
i read, "Detachment bahavior usually occurs when a value set outside of the data-binding" and that is the answer of my problem.
this thread can also be closed.