Error when using Datepicker

Error when using Datepicker

I'm creating a web application that uses datepicker in a module (colorbox widget for jquery).  When I load the content it loads in an iframe within the module. 

When I clicked on the field, it brings up the datepicker with no problem (The date that is set as the value of the input field is selected).  When I try to go forward a month or back a month, I get an error

Error: inst is null
Source File: js/jquery.ui.datepicker.js
Line: 1227

Here is the line of HTML
  1. <div id="date"><span class="label">Date:</span><span class="field"><input type="text" id="date" value="<?php echo $jobDate;?>" /></span></div>

Here is my javascript

  1. <script type="text/javascript" language="javascript" src="js/jquery.js"></script>
    <script type="text/javascript" language="javascript" src="js/jquery.ui.core.js"></script>
    <script type="text/javascript" language="javascript" src="js/jquery.ui.widget.js"></script>
    <script type="text/javascript" language="javascript" src="js/jquery.ui.datepicker.js"></script>



  3. <script type="text/javascript">
  4. $(document).ready(function() {
  5.     $("input#date").datepicker();
  6. });
  7. </script>

I am using jQuery 1.4.2 and jQuery UI 1.8.2

Anyone know what could be causing this issue?