Hi Jagjit,

Please refer below sample.

HTML

<table>
    <tr>
        <td>Added On:
            <asp:TextBox ID="txtAddedon" runat="server" Text="12/05/2019"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>Valid Upto:
            <asp:TextBox ID="txtValidUpTo" runat="server" Text="12/06/2022"></asp:TextBox>
        </td>
        <td>
            <asp:CompareValidator ID="cvDates" runat="server" ControlToValidate="txtValidUpTo"
                ControlToCompare="txtAddedon" ValidationGroup="ComDate" Operator="GreaterThan" Type="Date" ForeColor="Red"
                ErrorMessage="ValidTo must be greater then Addon Date."></asp:CompareValidator>
        </td>
    </tr>
    <tr>
        <td>
            <asp:Button ID="btnCompare" runat="server" Text="Compare" ValidationGroup="ComDate" />
        </td>
    </tr>
</table>

Screenshot

Reference

Compare dates in dd/mm/yyyy format using ASP.Net CompareValidator