I have the following code in the codebehind file of an ASP.Net page
txtStartDate.Text = DateTime.Today.ToString("MM-dd-yyyy");
That format string should work as expected. I'd check your textbox to make sure you don't have some sort of mask (AJAX MaskedEditExtender?) on it. If you did, and maybe had the mask incorrect, it could overwrite what you were putting in the textbox.