DateTime conversion bug?

Am happily coding along in VS2008 in C# 2.0 and I came along this weird bug here...

Assuming this.Value = "12/13/2007 12:00:00 PM"

if (this.Value != null)
{
textbox.Text = String.Format(this.Value.ToString(), "MM/dd/yyyy");
}

textbox.Text shows "12/13/2007 12:00:00 PM"!!!

If I do it this way:


if (this.Value != null)
{
textbox.Text = DateTime.Parse(this.Value.ToString()).ToString("MM/dd/yyyy");
}
textbox.Text now shows "12/13/2007", perfect...

Help me see why this didn't convert properly the first time?