jQuery Grid Plugin - jqGrid

Hi i downloaded from http://www.trirand.com/blog/?page_id=6 checking all components.

am using jqgrid with a date cell:

        jQuery("#jqGridWI").jqGrid({
            datatype: "local",
            colNames:['Order Date', 'Due Date', 'PON'],
            colModel:[
                {name:'orderdate', index:'orderdate', sorttype:'date'},
                {name:'duedate', index:'orderdate', sorttype:'date'},
                {name:'pon', index:'pon'}
                ],
            ...
            sortname: 'duedate',
            viewrecords: true,
            sortorder: "desc"});

the PON column sorts okay,

but when i try to sort either Order Date or Due Date get this error (via FireBug):

$.jgrid.formatter is undefined. FireFox v. 3.5.5

jqGrid version 3.6.1