Property-strange behaviour

I'm trying to use FormatDateTime function with TextBox.Text. TextBox displays 12/03/1999 12:00:00 AM. I'd like to have just 12/03/1999. I'm using this:
TextBox.Text=FormatDateTime(TextBox.Text, DateFormat.ShortDate). In debug mode I'm checking the value of TextBox.Text. Before execution it contains 12/03/1999 12:00:00 AM, after execution 12/03/1999. But on the screen I still see 12/03/1999 12:00:00 AM. What's going on there? Do I have to call
some method to refresh the graphical contents of the Text Box? Or what? I never seen any "broken links" between the property of control and its representation on the screen in previous versions of VB. Any idea what's going on in VB.Net? How it works now? I saw something similar with ComboBox.Text property before. I set it programmatically to "", I check it in code - it says that it's really blank, but I still see the previous contents on the screen.
Thank you

Vlad