Validate Date format when using Date CompareValidator in ASP.Net in GridView

You need to date validator too

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:RegularExpressionValidator ControlToValidate = "TextBox1" ID="RegularExpressionValidator1" ValidationExpression="^[0-9m]{1,2}/[0-9d]{1,2}/[0-9y]{4}$" 
 runat="server" ErrorMessage="Invalid Date. Enter date format as mm/dd/yyyy."></asp:RegularExpressionValidator>