diff --git a/addressbook/inc/class.addressbook_groupdav.inc.php b/addressbook/inc/class.addressbook_groupdav.inc.php
index 4b1114df5d..4bb124b7df 100644
--- a/addressbook/inc/class.addressbook_groupdav.inc.php
+++ b/addressbook/inc/class.addressbook_groupdav.inc.php
@@ -22,7 +22,7 @@ class addressbook_groupdav extends groupdav_handler
 	/**
 	 * bo class of the application
 	 *
-	 * @var addressbook_vcal
+	 * @var addressbook_bo
 	 */
 	var $bo;
 
@@ -307,6 +307,24 @@ class addressbook_groupdav extends groupdav_handler
 		return true;
 	}
 
+	/**
+	 * Query ctag for addressbook
+	 * 
+	 * @return string
+	 */
+	public function getctag($path,$user)
+	{
+		$filter = array();
+		// show addressbook of a single user?
+		if ($user && $path != '/addressbook/') $filter['contact_owner'] = $user;
+		// should we hide the accounts addressbook
+		if ($GLOBALS['egw_info']['user']['preferences']['addressbook']['hide_accounts']) $filter['account_id'] = null;
+		
+		$result = $this->bo->search(array(),'MAX(contact_modified) AS contact_modified','','','','','',$filter);
+		
+		return date('Y-m-d H:i:s',$result[0]['modified']);
+	}
+
 	/**
 	 * Get the handler and set the supported fields
 	 *
diff --git a/phpgwapi/inc/class.groupdav.inc.php b/phpgwapi/inc/class.groupdav.inc.php
index b10d050de4..703823bf14 100644
--- a/phpgwapi/inc/class.groupdav.inc.php
+++ b/phpgwapi/inc/class.groupdav.inc.php
@@ -48,6 +48,10 @@ class groupdav extends HTTP_WebDAV_Server
 	 * CardDAV namespace
 	 */
 	const CARDDAV = 'urn:ietf:params:xml:ns:carddav';
+	/**
+	 * Calendarserver namespace (eg. for ctag)
+	 */
+	const CALENDARSERVER = 'http://calendarserver.org/ns/';
 	/**
 	 * Realm and powered by string
 	 */
@@ -229,7 +233,7 @@ class groupdav extends HTTP_WebDAV_Server
 		{
 			if ($method != 'REPORT' && !$id)	// no self URL for REPORT requests (only PROPFIND) or propfinds on an id
 			{
-				$files['files'][] = array(
+				$files['files'][0] = array(
 		        	'path'  => '/'.$app.'/',
 					// KAddressbook doubles the folder, if the self URL contains the GroupDAV/CalDAV resourcetypes
 		        	'props' => $this->_properties($app,$app=='addressbook'&&strpos($_SERVER['HTTP_USER_AGENT'],'KHTML') !== false),
@@ -237,6 +241,12 @@ class groupdav extends HTTP_WebDAV_Server
 			}
 			if (!$options['depth'] && !$id)
 			{
+				// add ctag if handler implements it (only for depth 0)
+				if (method_exists($handler,'getctag'))
+				{
+					$files['files'][0]['props'][] = HTTP_WebDAV_Server::mkprop(
+						groupdav::CALENDARSERVER,'getctag',$handler->getctag($options['path'],$user));
+				}
 				return true;	// depth 0 --> show only the self url
 			}
 			return $handler->propfind($options['path'],$options,$files,$user,$id);