Object doesn't support this property or method

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PHP

jQuery

Hi i have a php modal form with datepicker control and i have done validation

Here is my coding

<script type="text/javascript" language="javascript">
var $j=jQuery.noConflict();
function ff()
{

$j('#txtUserName').datepicker({
   onSelect: function(dateText, inst) { 
       dateAsString = dateText; //the first parameter of this function
     var dateAsObject = $(this).datepicker( 'getDate' ); //the getDate method
	  //alert(dateAsObject);
	  
	  var vv=$j.datepicker.formatDate('yy-mm-dd', dateAsObject);
	  $j('#txtUserName').val(vv);
		
	  $j(this).datepicker( 'destroy' );
   }
});
} 

$j(document).ready(function(){

		$j("#form1").validate({
		
			debug: false,
			rules: {
				txtDate:{ 
				 
				required:true
				 
				
				},
				txtPassword: {
					required: true
					
				}
				}
				});
				
	});

but its giving error as object doesn't support this property or method.
How to solve this? Pls help