Cannot find textbox inside formview using popup calendar

Hello,
I am using a popup calendar to select dates. It works fine when I create records, but not so good when updating records, as updates are performed using a formview. To sum up what the javascript functions do, they get the day pressed on the popup calender and pass the value to the textbox on the parent page. The issue I am having is when a textbox is inside a template (in my case EditItemTemplate), the function can not find the date textbox. The following are code snippets of the function to call the calendar and the popup page containing the calendar:

Parent page
<asp:LinkButton ID="lbChooseDate" runat="server" OnClientClick="PopupPicker('DueDateTextBox', 220, 220)">Choose Date</asp:LinkButton> <script language=javascript> function PopupPicker(ctl,w,h) { ...