TextBox will not be updated periodically - wpf

Question

I have created a simple .net core wpf application to represent my problem. On the MainWindow there is a TextBox, which should be updated periodically via Binding. But the TextBox only shows 'Hello world' but not the time.
Thanks for any hints!
MainWindow.xaml:
<Window x:Class="WpfTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfTest"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Grid>
<TextBox Text="{Binding Path=DebugText}" TextWrapping="Wrap" Margin="10,191,10,192"/>
</Grid>
</Window>
MainWindow.xaml.cs:
using System.Windows;
namespace WpfTest
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private readonly ViewModel _viewModel = new ViewModel();
public MainWindow()
{
InitializeComponent();
DataContext = _viewModel;
}
}
}
ViewModel.cs:
using System;
using System.Threading;
using System.Windows;
using System.Windows.Controls;
namespace WpfTest
{
public class ViewModel : UserControl
{
private Timer _updateTimer;
public ViewModel()
{
DebugText = "Hello world";
AutoResetEvent timerEvent = new AutoResetEvent(true);
_updateTimer = new Timer(OnTimer, timerEvent, 1000, 1000);
}
private void OnTimer(object stateInfo)
{
Application.Current.Dispatcher.Invoke(() =>
{
DebugText = DateTime.Now.ToString();
});
}
public string DebugText
{
get => (string)GetValue(DebugTextProperty);
set => SetValue(DebugTextProperty, value);
}
public static readonly DependencyProperty DebugTextProperty =
DependencyProperty.Register("DebugText",
typeof(string),
typeof(MainWindow),
new PropertyMetadata(null));
}
}