Table Sorter is not working proprly

I am using JQuery Table Sorter...Its working fine for all the fields except the year field in my grid view...Year Field is not sorted properly

This is my code:

 $(function()
 {
   $("table[id$='gvInvoiceDetails']").tablesorter({ headers: { 4 : {sorter :
    "currency"}},textExtraction:"complex", widgets: ['zebra']
   }).tablesorterPager({container: $("#pager")});                            
});

Any Suggesstions,Ideas,Samplecode

Thanks

Check this out : Using jQuery tablesorter to sort mm/yy dates