new row in datagrid with custom data

Question

I have two datagrids. The first one is an overview about a time tracking. The second one shows some details.

For example: The first datagrid contains each day, where a employee works. The second datagrid contains the time stamping for a day.

However, when I add a new stamping row, the controls in the datagrid are empty. That is not really a problem but the datepicker shows the date 01/01/0001.

what i want is when i add a new row, the date field should be filled with the date from the first datagrid.

VIEW:

 <DataGrid Grid.Column="0"
                          IsReadOnly="True"
                          AutoGenerateColumns="False"
                          Grid.Row="0"
                          x:Name="DgStundenView"
                          Margin="0 0 10 0"
                          SelectedItem="{Binding SelectedItem}"
                          ItemsSource="{Binding TimeOverviewList}">
                <DataGrid.Columns>
                    <DataGridTextColumn Header="Wochentag" Binding="{Binding Wochentag}" />
                    <DataGridTextColumn Header="Datum" Binding="{Binding Tag}" />
                    <DataGridTextColumn Header="Arbeitszeit Soll" Binding="{Binding ArbeitszeitInStunden}" />
                    <DataGridTextColumn Header="Arbeitszeit gesamt" Binding="{Binding GesamtDauerInStunden}" />
                    <DataGridTextColumn Header="Pausenzeit" Binding="{Binding Pausenzeit}" />
                    <DataGridTextColumn Header="Pausendifferenz" Binding="{Binding PausenDifferenzInStunden}" />
                    <DataGridTextColumn Header="Arbeitszeit inkl. Pause" Binding="{Binding TatsaechlicheDauerInStunden}" />
                    <DataGridCheckBoxColumn Header="Status" Binding="{Binding StempelungVorhanden,Mode=OneWay}" />
                    <DataGridTextColumn Header="Info" Binding="{Binding Info}" />
                </DataGrid.Columns>

                <DataGrid.RowStyle>
                    <Style TargetType="DataGridRow">
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding Wochentag}" Value="Sa">
                                <Setter Property="Background" Value="LightGray" />
                            </DataTrigger>
                            <DataTrigger Binding="{Binding Wochentag}" Value="So">
                                <Setter Property="Background" Value="LightGray" />
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </DataGrid.RowStyle>
            </DataGrid>

            <DataGrid Grid.Column="0"
                      Grid.Row="1"
                          x:Name="DgStempelungen"
                          Margin="0 10 10 0"
                          AutoGenerateColumns="False"
                      CanUserAddRows="True"
                      SelectedItem="{Binding SelectedValue}"
                      ItemsSource="{Binding TimeDetailList}">


                <DataGrid.Resources>
                    <x:Array x:Key="Reasons" Type="system:String">
                        <system:String>NICHT ANWENDBAR</system:String>
                        <system:String>KRANK</system:String>
                        <system:String>GANZER TAG URLAUB</system:String>
                    </x:Array>
                </DataGrid.Resources>

                <DataGrid.Columns>

                    <DataGridTextColumn Header="ID" IsReadOnly="True" Binding="{Binding  Id}"/>
                    <DataGridTemplateColumn Header="KOMMEN DATUM">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <DatePicker SelectedDate="{Binding  ComeBooking}"/>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                    </DataGridTemplateColumn>
                    <DataGridTemplateColumn Header="KOMMEN UHRZEIT">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <materialDesign:TimePicker Text="{Binding ComeBookingTime, StringFormat=t}"
                                                           Is24Hours="True">
                                </materialDesign:TimePicker>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                    </DataGridTemplateColumn>
                    <DataGridTemplateColumn Header="GEHEN DATUM">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <DatePicker SelectedDate="{Binding GoBooking}"/>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                    </DataGridTemplateColumn>
                    <DataGridTemplateColumn Header="GEHEN UHRZEIT">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <materialDesign:TimePicker Text="{Binding GoBookingTime, StringFormat=t}"
                                                           Is24Hours="True">
                                </materialDesign:TimePicker>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                    </DataGridTemplateColumn>
                    <DataGridComboBoxColumn Header="GRUND"
                                            ItemsSource="{StaticResource Reasons}"
                                            TextBinding="{Binding Info}"/>

VIEW MODEL

   [CanBeNull] private ClassTimeTrackingDayStamp _selectedItem;

        [CanBeNull]
        public ClassTimeTrackingDayStamp SelectedItem
        {
            get { return _selectedItem; }
            set
            {
                _selectedItem = value;
                OnPropertyChanged();
                if (_selectedItem != null)
                    TimeDetailList = new ObservableCollection<ClassTimeTrackingTimeStamp>(_selectedItem.Stempelungen);
            }
        }

        private ObservableCollection<ClassTimeTrackingDayStamp> _timeOverviewList;

        public ObservableCollection<ClassTimeTrackingDayStamp> TimeOverviewList
        {
            get { return _timeOverviewList; }
            set
            {
                _timeOverviewList = value;
                OnPropertyChanged();
            }
        }

        [CanBeNull] private ObservableCollection<ClassTimeTrackingTimeStamp> _timeDetailList;

        [CanBeNull]
        public ObservableCollection<ClassTimeTrackingTimeStamp> TimeDetailList
        {
            get { return _timeDetailList; }
            set
            {
                _timeDetailList = value;
                OnPropertyChanged();
            }
        }

MODEL

   public class ClassTimeTrackingTimeStamp : ViewModelBase
    {
        public DateTime ComeBooking { get; set; }

        public DateTime GoBooking { get; set; }

        public TimeSpan ComeBookingTime { get; set; }
        public TimeSpan GoBookingTime { get; set; }

        public int Id { get; set; }

        public string Info { get; set; }
    }

Thank you for your help:)

Best regards Levin