WPF Binding to DependencyProperty of UserControl in DataTemplate is not working - wpf

Question

i have a custom userControl with DPs and have the problem that the binding to these properties only works if i use the controls outside of datatemplates.
Outside of Datatemplates works the usercontrol great.
XAML to Test the UserControl in a DataTemplate
<GroupBox Header="DataTemplate" Padding="5">
<ItemsControl ItemsSource="{Binding Dummies}">
<ItemsControl.ItemTemplate>
<DataTemplate DataType="local:Dummy">
<StackPanel Orientation="Horizontal" Margin="2">
<common:QuarterPicker SelectedFirstDay="{Binding Gueltigkeit}" Margin="5,0" />
<!--control the value of the item-->
<TextBlock Text="Gueltigkeit: "/>
<TextBlock Text="{Binding Gueltigkeit}"/>
</StackPanel>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</GroupBox>
MainWindow codebehind and ViewModel
public partial class QuarterPickerTest : UserControl
{
public QuarterPickerTest()
{
InitializeComponent();
this.DataContext = new ViewModel();
}
}
public class ViewModel
{
public ViewModel()
{
this.Dummy = new Dummy { Gueltigkeit = DateTime.Today };
this.Dummies = new List<Dummy>
{
new Dummy {Gueltigkeit = DateTime.Today},
new Dummy {Gueltigkeit = DateTime.Today.AddMonths(6)},
new Dummy {Gueltigkeit = DateTime.Today.AddDays(-6)},
};
}
public Dummy Dummy { get; set; }
public List<Dummy> Dummies { get; set; }
}
Here is the code behind of my UserControl
#region SelectedFirstDay
public DateTime SelectedFirstDay
{
get { return (DateTime)GetValue(SelectedFirstDayProperty); }
set { SetValue(SelectedFirstDayProperty, value); }
}
public static readonly DependencyProperty SelectedFirstDayProperty
= DependencyProperty.Register("SelectedFirstDay", typeof (DateTime), typeof (QuarterPicker),
new FrameworkPropertyMetadata(DateTime.Today, SelectedFirstDayPropertyChangedCallback) { BindsTwoWayByDefault = true });
private static void SelectedFirstDayPropertyChangedCallback(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
{
var quarterPicker = dependencyObject as QuarterPicker;
var date = (DateTime)args.NewValue;
var quarter = GetQuarter(date);
quarterPicker.UpdateProperties(date.Year, quarter);
if (date.Year == quarterPicker.LeftInfiniteYear && quarter == quarterPicker.LeftInfiniteQuarter)
quarterPicker.ShowPopupButtonLeftInfinite();
}
#endregion
Thanks for any suggestions!