$(document).ready(function() {
      $('#file_upload').uploadify({
        'swf'  			: 'uploadify.swf',
        'uploader'    	: 'uploadify.php',
		'method'   		: 'post',
		'formData'      : {'creationDate' : 0},
        'auto'      	: true,
		'multi'     	: true,
		'buttonText' 	: 'Bilder hochladen',
		'onUploadStart'		: function (file) {  $("#file_upload").uploadify("settings", "formData", {"creationDate": file.creationdate}); },
		'onUploadError'       : function (event,ID,errorObj) { alert(errorObj.type + ' Error: ' + errorObj.info); },
		'onUploadComplete'    : function(event, ID, response, data) { if(typeof(response)!="undefined" && response!="1") { alert(response); return true; } return true;  },
		'onQueueComplete' : function(event,data) { window.location='index.php' }	
      });
    });

you can then retrieve the value in your uploadify.php script with $_POST['creationDate']

use file.modificationdate if you want the modified date rather than the creationdate.