MainWindow.xaml

<Window x:Class="DatesApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:DatesApp"
        Title="Date App" Height="400" Width="450">
    <Grid Margin="10">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <StackPanel>
            <TextBlock Margin="0 5" FontSize="15">Дни:</TextBlock>
            <TextBox x:Name="DayBox" Height="20"/>
            <TextBlock Margin="0 5" FontSize="15">Месяца:</TextBlock>
            <TextBox x:Name="MonthBox" Height="20"/>
            <TextBlock Margin="0 5" FontSize="15">Года:</TextBlock>
            <TextBox x:Name="YearBox" Height="20"/>
            <Button Content="Установить" Click="SetDate" Margin="0 20 0 5" Padding="5"/>
            <Button Content="Добавить" Click="AddDate" Margin="0 5" Padding="5"/>
            <Button Content="Сбросить" Click="ResetDate" Margin="0 5" Padding="5"/>
        </StackPanel>

        <StackPanel Grid.Row="2">
            <TextBlock HorizontalAlignment="Center" x:Name="ResultText" FontSize="20" Margin="0 15"/>
        </StackPanel>
    </Grid>
</Window>