<TextBox x:Name="txtNumberPass"
                Grid.Column="1"
                         Grid.Row="0"
                 Text="{Binding NumberPass}"
                 Width="80"
                 Style="{StaticResource TextBoxStyle}">

                </TextBox>

                <DatePicker x:Name="txtDataPass"
                         Grid.Column="2"
                         Grid.Row="0"
                        
                         Text="{Binding DataPass}"
                         Width="100"
                         Height="25"
                         VerticalAlignment="Center"
                            HorizontalAlignment="Center"/>

Text="{Binding DataPass, StringFormat={} {0:yyyy.MM.dd}}"

 public class DateTimeToDateConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ((DateTime)value).ToString("yyyy-MM.dd");
        }


        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return null;
        }

xmlns:conv="clr-namespace:OMP.Converters"
   <Grid.Resources>
                    <conv:DateTimeToDateConverter x:Key="DateConverter" />
                </Grid.Resources>
Text="{Binding DataPass, Converter={StaticResource DateConverter}}"

SelectedDate="{Binding DataPass}"