<asp:CompareValidator runat="server" ID="cmpNumbers" ControlToValidate="Button1"
                ControlToCompare="TextBox1" Operator="LessThan" Type="Integer" ErrorMessage="The first number should be smaller than the second number!" />
            <tr>

ControlToValidate="Button1"

<asp:CustomValidator id="CustomValidator1" runat=server 
      ControlToValidate = "Button1"
      ErrorMessage = "You must enter at least 8 characters!"
   ClientValidationFunction="validateLength" >

function validatefun(oSrc, args) {
args.IsValid = if (Date(TextBox1) < Date(TextBox2))
}

<asp:CustomValidator id="CustomValidator1" runat=server 
      ControlToValidate = "TextBox1"
      ErrorMessage = "Error"
   ClientValidationFunction="validatefun" ValidationGroup="vg">

<asp:CustomValidator id="CustomValidator1" runat=server 
      ControlToValidate = "TextBox2"
      ErrorMessage = "Error"
   ClientValidationFunction="validatefun" ValidationGroup="vg">

<asp:Button ValidationGroup="vg" />

function validatefun(oSrc, args) {
if (Date(TextBox1) < Date(TextBox2))
    args.IsValid = true
else
    args.IsValid = false

}

<asp:TextBox runat="server" ID="TextBox1" ClientIDMode="Static" />
<asp:CustomValidator id="CustomValidator1" runat=server 
      ControlToValidate = "TextBox1"
      ErrorMessage = "Error"
   ClientValidationFunction="validatefun" ValidationGroup="vg">

<asp:TextBox runat="server" ID="TextBox2" ClientIDMode="Static" />
<asp:CustomValidator id="CustomValidator1" runat=server 
      ControlToValidate = "TextBox2"
      ErrorMessage = "Error"
   ClientValidationFunction="validatefun" ValidationGroup="vg">

<asp:Button runat="server" ID="Button1" ValidationGroup="vg" />

.
.
.
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js" type="text/javascript"></script>
<script type="text/javascript">    
 $(document).ready(function() {
    function validatefun(oSrc, args) {
        alert("*");
        args.IsValid = ($("#TextBox1").val() < $("#TextBox2").val())            
    }
    });
</script>
}

<asp:CompareValidator runat="server" ID="cmpNumbers" ControlToValidate="TextBox1"
                        ControlToCompare="TextBox2[SIZE=5][/SIZE]" Operator="LessThan" Type="Date" ValidationGroup="vg" ErrorMessage="The first date should be smaller than the second date!" />