Hi gokulram,
Please refer the below code.
<head runat="server">
<title></title>
<script type="text/javascript">
function ValidateYear(sender, args) {
var currentYear = (new Date()).getFullYear();
args.IsValid = (document.getElementById('txtYear').value != '' && parseInt(document.getElementById('txtYear').value) < currentYear);
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox runat="server" ID="txtYear" />
<asp:CustomValidator ControlToValidate="txtYear" ClientValidationFunction="ValidateYear"
ErrorMessage="Year should be less than current year !" runat="server" ValidationGroup="g"></asp:CustomValidator>
<br />
<asp:Button Text="Validate" runat="server" ID="btnValidate" ValidationGroup="g" />
</div>
</form>
</body>
Screenshot
