Ajax Calendar extender clientid is always null in Javascript, cant set property.

I have a common scenario,  a grid control with a starting and stopping date search filter .  The Stop date cant be before the start date.

Selecting a starting date triggers a server side script that sets the  StartDate property of the stopdate's extender, disabling dates before the start date.

I tried to do this in Javascript but cant set the startdate of the extender in javascript as it's javascript client id is null and textbox.Validators[] dosent show the calendar extender either. 

How do you do this in Javascript?

var endTB = document.getElementById('<%=filter_enddate_CalendarExtender.ClientID %>');

endTB.StartDate is undefined,  EndTB is Always null.

<asp:TextBoxrunat="server"ID="filter_startdate" AutoPostBack="True" OnTextChanged="FilterStartdateTextChanged"TabIndex="1"></asp:TextBox>

<AjaxControlToolkit:CalendarExtenderID="filter_startdate_CalendarExtender"runat="server"Enabled="True"TargetControlID="filter_startdate">   </AjaxControlToolkit:CalendarExtender>

<asp:TextBoxrunat="server"ID="filter_enddate"   TabIndex="2" ></asp:TextBox>

 <AjaxControlToolkit:CalendarExtenderID="filter_enddate_CalendarExtender"runat="server"Enabled="True"TargetControlID="filter_enddate">   </AjaxControlToolkit:CalendarExtender>

<asp:Buttonrunat="server"ID="filter_submit"Text="Apply"OnClick="FilterSubmitClick"TabIndex="3"/>

(server side)________________________

protectedvoid FilterStartdateTextChanged(object sender, EventArgs e)

     {            filter_enddate_CalendarExtender.StartDate =DateTime.Parse(filter_startdate.Text);

 protectedvoid Page_Load(object sender, EventArgs e)

  {

if (IsPostBack) return;

  filter_startdate.Text = filter_enddate.Text =DateTime.Today.ToShortDateString();

  filter_enddate_CalendarExtender.StartDate =DateTime.Parse(filter_startdate.Text);

   filter_startdate.Focus();

 }