Hi
Try this code i given below.
//Declare a text box which is hidden in aspx page.
<div style="display: none;">
<asp:TextBox ID="hdnTodaysDate" runat="server" Text="11/10/2011"></asp:TextBox>
</div>
In your page load, use this code in code behind.
// Here i am assigning current system date to the text box.
hdnTodaysDate .Text = DateTime.Now.ToString("MM/dd/yyyy");
Use the CompareValidator to compare your date with the given date.
use the below code in aspx page.
<asp:CompareValidator ID="CompareValidatorBookingDeadline" runat="server" ControlToCompare="hdnTodaysDate" ControlToValidate="txtDOB" ErrorMessage="Please enter valid date." Operator="LessThan" ValidationGroup="vgUserRegistration" Type="Date"></asp:CompareValidator>
Happy coding.