try this
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><asp:CompareValidator ID="CompareValidator1"
runat="server" ControlToValidate="TextBox2" Operator="GreaterThan" Display="Dynamic"
ValueToCompare="<%# DateTime.Today.ToShortDateString() %>" Type="Date" Text="<br>Invalid Start Date" />
//In the Page Load
Page.DataBind();