datepicker.formatDate "Object doesn't support this property or method"

datepicker.formatDate "Object doesn't support this property or method"

Hello,
 
I'm new to jquery and am experiencing a world of pain getting started.  I am attempting to use a datepicker within an ASP.NET application.  I'm just trying to get the correct behavior of the control working first. 
 
I have added the following script references and script block to my MasterPage:
 
 <link type="text/css" href="/_css/green/jquery-ui-1.8.5.custom.css" rel="Stylesheet" /> 
 <script type="text/javascript" src="/_scripts/jquery-1.4.2.min.js"></script>
 <script type="text/javascript" src="/_scripts/jquery-ui-1.8.5.custom.min.js"></script>
 <script type="text/javascript" src="/_scripts/jquery.ui.datepicker-en-GB.js"></script>
 <script type="text/javascript" src="/_scripts/jquery.ui.datepicker-ext.js"></script>
 
 <script type="text/javascript">
  $(function() {
   $.datepicker.setDefaults($.datepicker.regional['en-GB']);
   $.datepicker.setDefaults({
    dateFormat: 'dd-M-y',
    changeMonth: true,
    changeYear: true,
    constrainInput: false,
    onClose: datePickerOnCloseExt,
   });
  });
 </script>

 
When a date is selected via the datepicker, the format is correctly displayed in the format dd-M-y.
 
However, if the user enters a date freehand (not via the datepicker) I would like to parse the input and rewrite it in the date input field in the format dd-M-y.
 
So, in my jquery.ui.datepicker-ext.js I have a method called datePickerOnCloseExt which is called onClose of the datepicker.  It looks like this:
 
  1. function datePickerOnCloseExt(dateText, inst) {
     if (dateText != '') {
      var formattedDate = $.datepicker.formatDate('dd-M-y', dateText);
      $(this).val(formattedDate);
     }
    }




 
When I run the page that contains the datepicker, I get a runtime error "Object doesn't support this property or method" on the line above where $.datepicker.formatDate() is called.  I cannot work out what I am doing wrong.  Can anyone please show me the error of my ways?
 
Many thanks