我还是解决了这个问题。我不会公布整个代码，我只会公布重大的变化。首先，自定义控件再次出现，只是非常简约。

public class ModernDatePicker : DatePicker, IDisposable
{
    public static readonly DependencyProperty VisualizatedDateProperty = DependencyProperty.Register(nameof(VisualizatedDate), typeof(DateTime), typeof(ModernDatePicker),
                           new FrameworkPropertyMetadata(DateTime.Now, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

    public DateTime VisualizatedDate
    {
        get { return (DateTime)GetValue(VisualizatedDateProperty); }
        set { SetValue(VisualizatedDateProperty, value); }
    }

    static ModernDatePicker()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(ModernDatePicker), new FrameworkPropertyMetadata(typeof(ModernDatePicker)));
    }

    public ModernDatePicker()
    {
        Loaded += ModernDatePicker_Loaded;
        SelectedDateChanged += ModernDatePicker_SelectedDateChanged;
    }

    private void ModernDatePicker_Loaded(object sender, RoutedEventArgs e)
    {
        if (this.SelectedDate == null)
        {
            SelectedDate = VisualizatedDate;
        }
    }

    private void ModernDatePicker_SelectedDateChanged(object? sender, SelectionChangedEventArgs e)
    {
        ModernDatePicker mdp = (ModernDatePicker)sender;
        if (mdp.SelectedDate != null)
        {
            VisualizatedDate = mdp.SelectedDate.Value;
        }
    }

}

    public void Dispose()
    {
        SelectedDateChanged -= ModernDatePicker_SelectedDateChanged;
        Loaded -= ModernDatePicker_Loaded;
    }

SelectDate是一个DateTime?类型，它不允许你在启动时立即获得值，你必须使用一个DependencyProperty，一个DateTime类型，以便能够使用绑定。

<Style TargetType="{x:Type cc:ModernDatePicker}">
        <Style.Resources>
            <conv:MonthToGenitiveMonthNameConverter x:Key="MonthToGenitiveMonthNameConverter"/>
            <conv:DayToDayNameConverter x:Key="DayToDayNameConverter"/>
            <conv:DateToTwoDigitDateConverter x:Key="DateToTwoDigitDateConverter"/>
        </Style.Resources>
        <Setter Property="Foreground" Value="#223266"/>
        <Setter Property="IsTodayHighlighted" Value="True"/>
        <Setter Property="CalendarStyle" Value="{DynamicResource ModernCalendarStyle}"/>
        <Setter Property="SelectedDateFormat" Value="Short"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="SubForeground" Value="#9eabe2"/>
        <Setter Property="OtherForeground" Value="White"/>
        <Setter Property="Padding" Value="2"/>
        <Setter Property="BorderBrush" Value="Transparent"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type cc:ModernDatePicker}">
                    <Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
                        <Grid x:Name="PART_Root" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                            <Grid  HorizontalAlignment="Stretch">
                                <StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
                                    <TextBlock Margin="5 0 0 0" Foreground="{TemplateBinding OtherForeground}" FontSize="50" Text="{Binding VisualizatedDate.Day, RelativeSource={RelativeSource AncestorType={x:Type cc:ModernDatePicker}},Converter = {StaticResource DateToTwoDigitDateConverter}}" />
                                    <StackPanel Margin="10,2,0,2" VerticalAlignment="Center" >
                                        <TextBlock Foreground="{TemplateBinding OtherForeground}" FontSize="24" FontWeight="Bold" Text="{Binding VisualizatedDate.Month, RelativeSource={RelativeSource AncestorType={x:Type cc:ModernDatePicker}}, Converter={StaticResource MonthToGenitiveMonthNameConverter}}"/>
                                        <TextBlock Foreground="{TemplateBinding SubForeground}" FontSize="14" Text="{Binding VisualizatedDate.DayOfWeek, RelativeSource={RelativeSource AncestorType={x:Type cc:ModernDatePicker}}, Converter={StaticResource DayToDayNameConverter}}"/>
                                    </StackPanel>
                                    <TextBlock Margin="5 0 0 0" Foreground="{TemplateBinding OtherForeground}" FontSize="50" Text="{Binding VisualizatedDate.Year, RelativeSource={RelativeSource AncestorType={x:Type cc:ModernDatePicker}}}" />
                                    <TextBlock Margin="5 0 0 0" Foreground="{TemplateBinding OtherForeground}" FontSize="50" Text="г." />
                                </StackPanel>
                                <StackPanel Orientation="Horizontal" HorizontalAlignment="Right">
                                    <Button Name="PART_Button" Style="{StaticResource ExtendedDataPickerButtonStyle}">
                                        <fa:IconImage Icon="Calendar"  Style="{StaticResource BigIconStyle}"/>
                                    </Button>
                                    <DatePickerTextBox x:Name="PART_TextBox" Visibility="Collapsed"/>
                                    <Popup x:Name="PART_Popup" AllowsTransparency="True" Placement="Bottom" PlacementTarget="{Binding ElementName=PART_Button}" StaysOpen="False" 
                                           ui:PopupProperties.HorizontalPlacementAlignment="Center"
                                           ui:PopupProperties.VerticalOffset="2"/>
                                </StackPanel>
                            </Grid>
                        </Grid>
                    </Border>
                </ControlTemplate>
            </Setter.Value>

事实上，通过类比