Custom WPF DatePickerTextBox Template

Try this out:

<DatePicker>
    <DatePicker.Resources>
        <Style TargetType="{x:Type DatePickerTextBox}">
            <Setter Property="Control.Template">
                <Setter.Value>
                    <ControlTemplate>
                        <TextBox x:Name="PART_TextBox" 
                                    Text="{Binding Path=SelectedDate, RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </DatePicker.Resources>
</DatePicker>

I realize this has been answered for a long time now, but binding directly to the DatePicker's Text property will allow the TextBox in your control template to easily honor the Short/Long format provided by the DatePicker.

<DatePicker>
    <DatePicker.Resources>
        <Style TargetType="{x:Type DatePickerTextBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate>
                        <TextBox Text="{Binding Text, RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </DatePicker.Resources>
</DatePicker>

The "PART_TextBox" is also not necessary because it is not part of the DatePickerTextBox template. The only PARTs that the DatePickerTextBox contains are:

[TemplatePart(Name = DatePickerTextBox.ElementContentName, Type = typeof(ContentControl))]
public sealed partial class DatePickerTextBox : TextBox

private const string ElementContentName = "PART_Watermark";

and inherited from TextBoxBase...

[TemplatePart(Name = "PART_ContentHost", Type = typeof(FrameworkElement))] 
public abstract class TextBoxBase : Control

internal const string ContentHostTemplateName = "PART_ContentHost";

Alternative Solution: If you opt out of using the TextBox and use the inherited PART you will be able to alter the DatePickerTextBox without altering the default functionality of the control.

<DatePicker>
    <DatePicker.Resources>
        <Style TargetType="{x:Type DatePickerTextBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate>
                        <Grid SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}">
                            <Border BorderThickness="{TemplateBinding BorderThickness}"
                                    BorderBrush="{TemplateBinding BorderBrush}"
                                    Background="{TemplateBinding Background}"/>

                            <ScrollViewer Name="PART_ContentHost"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </DatePicker.Resources>
</DatePicker>