Range validator problem in C#.NET to check date range.

I am having a problem with a range validator which I had asked a question about a few weeks ago.  Here is what is happening, and I actually created a simple ASP.NET page to test this with a textbox and a rangevalidator.  I set the maximum value of the rangevalidator but it DOES NOT pop up a warning if I put in todays today, which it should.  What am I doing wrong?

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        txtToDate.Text = DateTime.Now.AddDays(-1).ToShortDateString();  //Default to yesterday's date.
        RangeValidator1.MaximumValue = DateTime.Now.AddDays(-1).ToShortDateString();

    }
}