Hi,
You can use this javascript
function CheckValid() {
var fromDate = $('#<%=txtFromDt.ClientID %>').val();
var toDate = $('#<%= txtTodt.ClientID %>').val();
if ((fromDate != "" && toDate != "") && Date.parse(fromDate) > Date.parse(toDate)) {
$('#<%= lblErrorMsg.ClientID %>').text('To date should be greater than From date.');
return false;
}
}