Textbox Textchanged event doesn't fire...

Hi There,
I have two textboxes and two image buttons for the date selections. The two image buttons when cliked open a window (one at a time) which has the calendar.aspx page and this window returns the value of the date selection made and populates it into the textboxes. My ques is the textchanged event doesn't fire for the second text box. Also the textchanged event fires only after the second image button is clicked. Can anybody please suggest me how to deal this problem?

This is the code.which I am working on ..

StartDateCalendar, EndDate Calendar are two image butons...txtStartDate, txtEndDate are two textboxes

---aspx---
<asp:textbox id=txtStartDate runat="server" AutoPostBack="True" ReadOnly="true"  OnTextChanged=txtStartDate_TextChanged></asp:textbox>
<asp:imagebutton id=StartDateCalendar ImageUrl="../../../Resources/Images/Calendar.gif" CausesValidation="false"></asp:imagebutton>
<asp:textbox id=txtEndDate runat="server" AutoPostBack="True" ReadOnly="true"  OnTextChanged=txtEndDate_TextChanged></asp:textbox>
<asp:imagebutton id=EndDateCalendar runat="server" ImageUrl="../../../Resources/Images/Calendar.gif" CausesValidation="false"></asp:imagebutton>

--aspx.cs
StartDateCalendar.Attributes["onclick"] = "window.open('../Calendar.aspx?textbox=txtStartDate','cal','width=200,height=200,left=70,top=80');";
EndDateCalendar.Attributes["onclick"] = "window.open('../Calendar.aspx?textbox=txtEndDate','cal','width=200,height=200,left=70,top=80');";


----In calendar.aspx.cs...
//Hidden control
control.Value = Request.QueryString["textbox"].ToString();

private void calDate_SelectionChanged(object sender, System.EventArgs e)
            {
                  //System.DateTime dtDate = calDate.SelectedDate;
                  string strScript = "<script>window.opener.document.forms(0)." + control.Value + ".value = '";
                  strScript += calDate.SelectedDate.ToString("MM/dd/yyyy");
                  strScript += "';self.close()";
                  strScript += "</" + "script>";
                  this.RegisterClientScriptBlock("anything",strScript);
                  

}

Thanks a lot