jQueryui datepicker how delete lasts characters and set val to input

$( "#format" ).change(function() {
	$( "#datepicker" ).datepicker( "option", "dateFormat", $( this ).val() );
	
	if ($( "#format" ) == "yy MM dd") {
	$( "#format" ).click(function () 	{
	var inputString = $(document.getElementById ("#datepicker")).val();
  	var shortenedString = inputString.substr(0,(inputString.length -2));
	$( "#datepicker" ).val(shortenedString);
			});
                  }
	
		});

<p>Date: <input type="text" id="datepicker" size="30"></input>
<p>Format options:<br>
<select id="format">
<option value="mm/dd/yy">Default - mm/dd/yy</option>
<option value="yy-mm-dd">ISO 8601 - yy-mm-dd</option>
<option value="d M, y">Short - d M, y</option>
<option value="yy MM d">Medium - yy MM d</option>
<option value="DD, d MM, yy">Full - DD, d MM, yy</option>
<option value="yy MM dd">Year mounth</option>
</select>
</br>