Javacript date control not wotking on production server

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Javascript

ASP.NET2

I have date picker user control which is placed on master page. when i run application on local machine popup appears to select date and on production server i was able to select date in IE. Now all of sudden date popup is not coming on production server when i debug that application on production server it gives uncaught exception.I get exception in below javascript

var formname = document.getElementById(txtBox).form.name; 

I get nothing in formname variable but same is working before and on local machine. difference i found was ctl00_ prefix was coming on local machine while on production server ctl00 prefix is not coming now so what could be the reason?
I am using ASP.net 2.0 framework. below is my code
Asp.net code :

<asp:textbox id="tbDate" runat="server" Width="104px" CssClass="clsTextBox">        
        <asp:Image  id="imgCal" Visible="True" runat="server"  ImageUrl="../images/calendar.jpg"/>

Code Behind:

protected override void OnPreRender(EventArgs e)
{
  //  base.OnPreRender(e);
    tbDate.Attributes.Add("onkeypress", "javascript:return WriteProtect()");
    imgCal.ImageUrl = "../images/calendar.jpg";

    imgCal.Attributes.Add("onclick", "javascript:cbfshowcalendar('" + imgCal.ClientID + "','" + tbDate.ClientID + "');event.cancelBubble=true;");

    if(!Page.ClientScript.IsStartupScriptRegistered("CalendarJS"))
    {
        Page.ClientScript.RegisterStartupScript(this.GetType(), "CalendarJS", "<script type='text/javascript'  src='../include/calendar.js'></script>");
    }
}

Javascript :

function cbfshowcalendar(imgObj, txtBox)
{
//The current size of the IFRAME is width=167 and height=185
//var x=eval("document.all." + imgname + ".offsetLeft");
//var y=eval("document.all." + imgname + ".offsetTop");
var objname = txtBox;
var imgname =  imgObj;

var formname = document.getElementById(txtBox).form.name; -- gets nothing in formname 


var x=findPosX(eval("document.all." + imgname));
var y=findPosY(eval("document.all." + imgname));
var max_x = window.screen.width;
var max_y = window.screen.height-100;
. . . .
function cbfloadmefirst(formname, objname)
{
var tempObj;
var tempDate;

tempObj = eval("document." + formname + "." + objname);  -- error on this line
Tags

What I have tried:

i found was ctl00_ prefix was coming on local machine while on production server ctl00 prefix is not coming now so what could be the reason?