diff --git a/application/controllers/Test.php b/application/controllers/Test.php
index 0454d16f4..f3803bdae 100644
--- a/application/controllers/Test.php
+++ b/application/controllers/Test.php
@@ -22,12 +22,12 @@ class Test extends FHC_Controller
 	{
        
         $authBackend = new SabreDAVAuthLib();
-        $principalBackend = new SabreDAVACLPrincipalBackendLib($authBackend);
-        $calendarBackend = new SabreDAVCalDAVBackendLib($authBackend);
+        $principalBackend = new SabreDAVACLPrincipalBackend($authBackend);
+        $calendarBackend = new SabreDAVCalDAVBackend($authBackend);
 
         $tree = array(
             new \Sabre\CalDAV\Principal\Collection($principalBackend),
-            new SabreDAVReadOnlyCalendarRootLib($principalBackend, $calendarBackend)
+            new SabreDAVReadOnlyCalendarRoot($principalBackend, $calendarBackend)
         );
 
         $server = new \Sabre\DAV\Server($tree);
@@ -39,8 +39,6 @@ class Test extends FHC_Controller
         $authPlugin = new \Sabre\DAV\Auth\Plugin($authBackend);
         $server->addPlugin($authPlugin);
 
-        $this->_registerReadOnlyWriteInterceptor($server);
-
         $caldavPlugin = new \Sabre\CalDAV\Plugin();
         $server->addPlugin($caldavPlugin);
 
@@ -52,94 +50,4 @@ class Test extends FHC_Controller
 
         $server->exec();
 	}
-
-	private function _registerReadOnlyWriteInterceptor($server)
-	{
-		$callback = function() use ($server) {
-			return $this->_discardReadOnlyWriteRequest($server, func_get_args());
-		};
-
-		if (method_exists($server, 'on'))
-		{
-			$server->on('beforeMethod', $callback, 20);
-			return;
-		}
-
-		if (method_exists($server, 'subscribeEvent'))
-		{
-			$server->subscribeEvent('beforeMethod', $callback, 20);
-		}
-	}
-
-	private function _discardReadOnlyWriteRequest($server, array $eventArgs)
-	{
-		$method = $this->_getDavRequestMethod($eventArgs);
-		if (!in_array($method, array('PUT', 'DELETE', 'PROPPATCH', 'MKCOL', 'MOVE', 'COPY'), true))
-			return true;
-
-		$this->_sendReadOnlyWriteAcceptedResponse($server, $eventArgs);
-		return false;
-	}
-
-	private function _getDavRequestMethod(array $eventArgs)
-	{
-		foreach ($eventArgs as $argument)
-		{
-			if (is_string($argument))
-				return strtoupper($argument);
-
-			if (is_object($argument) && method_exists($argument, 'getMethod'))
-				return strtoupper($argument->getMethod());
-		}
-
-		return isset($_SERVER['REQUEST_METHOD']) ? strtoupper($_SERVER['REQUEST_METHOD']) : '';
-	}
-
-	private function _sendReadOnlyWriteAcceptedResponse($server, array $eventArgs)
-	{
-		$response = $this->_getDavResponse($server, $eventArgs);
-
-		if (is_object($response))
-		{
-			if (method_exists($response, 'setStatus'))
-			{
-				$response->setStatus(204);
-				if (method_exists($response, 'setHeader'))
-					$response->setHeader('Content-Length', '0');
-				if (method_exists($response, 'setBody'))
-					$response->setBody('');
-				return;
-			}
-
-			if (method_exists($response, 'sendStatus'))
-			{
-				if (method_exists($response, 'setHeader'))
-					$response->setHeader('Content-Length', '0');
-				$response->sendStatus(204);
-				return;
-			}
-		}
-
-		if (!headers_sent())
-		{
-			header('HTTP/1.1 204 No Content');
-			header('Content-Length: 0');
-		}
-	}
-
-	private function _getDavResponse($server, array $eventArgs)
-	{
-		foreach ($eventArgs as $argument)
-		{
-			if (is_object($argument) && (
-				method_exists($argument, 'setStatus')
-				|| method_exists($argument, 'sendStatus')
-			))
-			{
-				return $argument;
-			}
-		}
-
-		return isset($server->httpResponse) ? $server->httpResponse : null;
-	}
 }
diff --git a/application/libraries/sabredav/SabreDAVCalDAVLib.php b/application/libraries/sabredav/SabreDAVCalDAVLib.php
index e79f8a6e6..0417f7002 100644
--- a/application/libraries/sabredav/SabreDAVCalDAVLib.php
+++ b/application/libraries/sabredav/SabreDAVCalDAVLib.php
@@ -1,971 +1,16 @@
 <?php
 
+require_once __DIR__.'/caldav/SabreDAVACLPrincipalBackend.php';
+require_once __DIR__.'/caldav/SabreDAVCalDAVBackend.php';
+require_once __DIR__.'/caldav/SabreDAVReadOnlyACL.php';
+require_once __DIR__.'/caldav/SabreDAVReadOnlyCalendarRoot.php';
+require_once __DIR__.'/caldav/SabreDAVReadOnlyCalendarHome.php';
+require_once __DIR__.'/caldav/SabreDAVReadOnlyCalendar.php';
+require_once __DIR__.'/caldav/SabreDAVReadOnlyCalendarObject.php';
+
 /**
- * SabreDAV CalDAV classes for the LVPlan calendar endpoint.
+ * Loads the SabreDAV CalDAV classes for the LVPlan calendar endpoint.
  */
 class SabreDAVCalDAVLib
 {
 }
-
-/**
- * PDO principal backend
- *
- * This is a simple principal backend that maps exactly to the users table, as
- * used by Sabre_DAV_Auth_Backend_PDO.
- *
- * It assumes all principals are in a single collection. The default collection
- * is 'principals/', but this can be overriden.
- *
- * @package Sabre
- * @subpackage DAVACL
- * @copyright Copyright (C) 2007-2011 Rooftop Solutions. All rights reserved.
- * @author Evert Pot (http://www.rooftopsolutions.nl/)
- * @license http://code.google.com/p/sabredav/wiki/License Modified BSD License
- */
-class SabreDAVACLPrincipalBackendLib implements \Sabre\DAVACL\PrincipalBackend\BackendInterface
-{
-    /**
-     * PDO table name for 'principals'
-     *
-     * @var string
-     */
-    protected $tableName;
-
-    /**
-     * PDO table name for 'group members'
-     *
-     * @var string
-     */
-    protected $groupMembersTableName;
-
-	protected $result_ma;
-	protected $auth;
-    /**
-     * Sets up the backend.
-     *
-     * @param PDO $pdo
-     * @param string $tableName
-     */
-    public function __construct($auth)
-	{
-		$this->auth = $auth;
-
-		/*
-		$ma = new mitarbeiter();
-		$this->result_ma = $ma->getMitarbeiter(null,null,null);
-		*/
-    }
-
-	/**
-	 * Liefert den eingeloggten User
-	 */
-	function getUser()
-	{
-		return $this->auth->getCurrentUser();
-	}
-
-
-    /**
-     * Returns a list of principals based on a prefix.
-     *
-     * This prefix will often contain something like 'principals'. You are only
-     * expected to return principals that are in this base path.
-     *
-     * You are expected to return at least a 'uri' for every user, you can
-     * return any additional properties if you wish so. Common properties are:
-     *   {DAV:}displayname
-     *   {http://sabredav.org/ns}email-address - This is a custom SabreDAV
-     *     field that's actualy injected in a number of other properties. If
-     *     you have an email address, use this property.
-     *
-     * @param string $prefixPath
-     * @return array
-     */
-    public function getPrincipalsByPrefix($prefixPath)
-	{
-		//$prefixPath = principals
-		//error_log('Principal.php/getPrincipalsByPrefix('.$prefixPath.')');
-        $principals = array();
-		$user = $this->getUser();
-
-		if($prefixPath=='principals')
-		{
-
-		    $principals[] = array(
-					'id' => $user,
-		            'uri' => 'principals/'.$user,
-		            '{DAV:}displayname' => $user,
-		            '{http://sabredav.org/ns}email-address' => $user.'@example.com',
-		        );
-		}
-
-        return $principals;
-
-    }
-
-    /**
-     * Returns a specific principal, specified by it's path.
-     * The returned structure should be the exact same as from
-     * getPrincipalsByPrefix.
-     *
-     * @param string $path
-     * @return array
-     */
-    public function getPrincipalByPath($path)
-	{
-		$user = mb_substr($path,11);
-
-        $result = array(
-            'id'  => $user,
-            'uri' => 'principals/'.$user,
-            '{DAV:}displayname' => $user,
-            '{http://sabredav.org/ns}email-address' => $user.'@example.com',
-        );
-
-		return $result;
-    }
-
-    /**
-     * Returns the list of members for a group-principal
-     *
-     * @param string $principal
-     * @return array
-     */
-    public function getGroupMemberSet($principal)
-	{
-        $result = array();
-
-		return $result;
-    }
-
-    /**
-     * Returns the list of groups a principal is a member of
-     *
-     * @param string $principal
-     * @return array
-     */
-    public function getGroupMembership($principal)
-	{
-		$result = array();
-		if(preg_match('/^principals\/[0-9A-Za-z\-]*$/',$principal))
-		{
-			$user = mb_substr($principal,11);
-		}
-        return $result;
-
-    }
-
-    /**
-     * Updates the list of group members for a group principal.
-     *
-     * The principals should be passed as a list of uri's.
-     *
-     * @param string $principal
-     * @param array $members
-     * @return void
-     */
-    public function setGroupMemberSet($principal, array $members)
-	{
-		throw new \Sabre\DAV\Exception('Not implemented');
-    }
-
-   public function updatePrincipal($path, \Sabre\DAV\PropPatch $propPatch)
-   {
-	$propPatch->setRemainingResultCode(403);
-   }
-
-	public function searchPrincipals($prefixPath, array $searchProperties, $test = 'allof')
-	{
-		return array();
-	}
-
-	public function findByUri($uri, $principalPrefix)
-	{
-		return null;
-	}
-}
-
-/**
- * CalDAV backend
- */
-class SabreDAVCalDAVBackendLib extends \Sabre\CalDAV\Backend\AbstractBackend
-{
-	protected $auth;
-
-	protected $CI;
-
-	/**
-     * Creates the backend
-     *
-     * @param AuthBackend $auth
-     */
-    public function __construct($auth)
-	{
-		$this->auth = $auth;
-
-		$this->CI =& get_instance();
-    }
-
-	/**
-	 * Liefert den eingeloggten User
-	 */
-	function getUser()
-	{
-		return $this->auth->getCurrentUser();
-	}
-
-    /**
-     * Returns a list of calendars for a principal.
-     *
-     * Every project is an array with the following keys:
-     *  * id, a unique id that will be used by other functions to modify the
-     *    calendar. This can be the same as the uri or a database key.
-     *  * uri, which the basename of the uri with which the calendar is
-     *    accessed.
-     *  * principalUri. The owner of the calendar. Almost always the same as
-     *    principalUri passed to this method.
-     *
-     * Furthermore it can contain webdav properties in clark notation. A very
-     * common one is '{DAV:}displayname'.
-     *
-     * @param string $principalUri
-     * @return array
-     */
-    public function getCalendarsForUser($principalUri)
-	{
-		$user = mb_substr($principalUri,11);
-        $calendars = array();
-		$calendar = array(
-			'id' => $user,
-			'uri' => 'LVPlan-'.$user,
-			'principaluri' => 'principals/'.$user,
-			'{' . \Sabre\CalDAV\Plugin::NS_CALENDARSERVER . '}getctag' => $this->buildCalendarCtag($user),
-			'{' . \Sabre\CalDAV\Plugin::NS_CALDAV . '}supported-calendar-component-set' => new \Sabre\CalDAV\Xml\Property\SupportedCalendarComponentSet(array('VEVENT')),
-			'{DAV:}displayname'                          => 'LVPlan',
-			'{urn:ietf:params:xml:ns:caldav}calendar-description' => 'description comes here',
-			'{urn:ietf:params:xml:ns:caldav}calendar-timezone'    => 'Europe/Vienna',
-			'{http://apple.com/ns/ical/}calendar-order'  => '1',
-			'{http://apple.com/ns/ical/}calendar-color'  => '#FF0000',
-			'{http://sabredav.org/ns}read-only' => 1
-        );
-		$calendars[] = $calendar;
-
-        return $calendars;
-    }
-
-    /**
-     * Creates a new calendar for a principal.
-     *
-     * If the creation was a success, an id must be returned that can be used to reference
-     * this calendar in other methods, such as updateCalendar
-     *
-     * @param string $principalUri
-     * @param string $calendarUri
-     * @param array $properties
-     */
-    public function createCalendar($principalUri,$calendarUri, array $properties)
-	{
-		return $calendarUri;
-    }
-
-    /**
-     * Updates properties for a calendar.
-     *
-     * The mutations array uses the propertyName in clark-notation as key,
-     * and the array value for the property value. In the case a property
-     * should be deleted, the property value will be null.
-     *
-     * This method must be atomic. If one property cannot be changed, the
-     * entire operation must fail.
-     *
-     * If the operation was successful, true can be returned.
-     * If the operation failed, false can be returned.
-     *
-     * Deletion of a non-existant property is always succesful.
-     *
-     * Lastly, it is optional to return detailed information about any
-     * failures. In this case an array should be returned with the following
-     * structure:
-     *
-     * array(
-     *   403 => array(
-     *      '{DAV:}displayname' => null,
-     *   ),
-     *   424 => array(
-     *      '{DAV:}owner' => null,
-     *   )
-     * )
-     *
-     * In this example it was forbidden to update {DAV:}displayname.
-     * (403 Forbidden), which in turn also caused {DAV:}owner to fail
-     * (424 Failed Dependency) because the request needs to be atomic.
-     *
-     * @param string $calendarId
-     * @param array $mutations
-     * @return bool|array
-     */
-    public function updateCalendar($calendarId, \Sabre\DAV\PropPatch $propPatch)
-	{
-        $propPatch->setRemainingResultCode(200);
-    }
-
-    /**
-     * Delete a calendar and all it's objects
-     *
-     * @param string $calendarId
-     * @return void
-     */
-    public function deleteCalendar($calendarId)
-	{
-		return null;
-    }
-
-
-	public function getCalendarData($userUID, $objectUri=null)
-	{
-		$this->CI->load->model("ressource/Mitarbeiter_model", "MitarbeiterModel");
-		$this->CI->load->model("person/Benutzer_model", "BenutzerModel");
-
-		$user = $this->CI->BenutzerModel->loadWhere(array("uid" => $userUID));
-		if(!$user)
-			die('User invalid');
-
-		$this->CI->load->library('KalenderLib', ['uid' => $userUID]);
-
-		$isUserEmployeeResult = $this->CI->MitarbeiterModel->isMitarbeiter($userUID);
-		if (isError($isUserEmployeeResult)) {
-			return;
-		}
-		$isUserEmployee = getData($isUserEmployeeResult);
-
-		$startDate = date('Y-m-d', mktime(0,0,0,date('m'),date('d')-14,date('Y')));
-		$endDate = date('Y-m-d', mktime(0,0,0,date('m')+6,date('d'),date('Y')));
-
-		$data = [];
-
-		if ($isUserEmployee) {
-			$data = $this->CI->kalenderlib->getPlanForLecturerByLecturer($startDate, $endDate, $userUID);
-		} else {
-			$data = $this->CI->kalenderlib->getPlanForStudentByStudent($startDate, $endDate, $userUID);
-		}
-
-		if(!is_array($data))
-			$data = array();
-
-		foreach($data as $item)
-		{
-			$item->calendarDataFragment = $this->buildCalendarDataFragment($item);
-		}
-
-		if (is_null($objectUri))
-		{
-			return $data;
-		}
-
-		$normalizedObjectUri = $this->normalizeObjectUri($objectUri);
-		foreach($data as $row)
-		{
-			if(md5($row->eindeutige_gruppen_id) == $normalizedObjectUri)
-			{
-				return $row;
-			}
-		}
-
-		return null;
-	}
-
-	public function makeCal($event)
-	{
-		return $this->buildICalLine('BEGIN', 'VCALENDAR')
-			.$this->buildICalLine('VERSION', '2.0')
-			.$this->buildICalTextLine('PRODID', 'FH Technikum Wien')
-			.$this->buildICalLine('BEGIN', 'VTIMEZONE')
-			.$this->buildICalLine('TZID', 'Europe/Vienna')
-			.$this->buildICalLine('BEGIN', 'DAYLIGHT')
-			.$this->buildICalLine('TZOFFSETFROM', '+0100')
-			.$this->buildICalLine('RRULE', 'FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU')
-			.$this->buildICalLine('DTSTART', '19810329T020000')
-			.$this->buildICalLine('TZNAME', 'GMT+02:00')
-			.$this->buildICalLine('TZOFFSETTO', '+0200')
-			.$this->buildICalLine('END', 'DAYLIGHT')
-			.$this->buildICalLine('BEGIN', 'STANDARD')
-			.$this->buildICalLine('TZOFFSETFROM', '+0200')
-			.$this->buildICalLine('RRULE', 'FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU')
-			.$this->buildICalLine('DTSTART', '19961027T030000')
-			.$this->buildICalLine('TZNAME', 'GMT+01:00')
-			.$this->buildICalLine('TZOFFSETTO', '+0100')
-			.$this->buildICalLine('END', 'STANDARD')
-			.$this->buildICalLine('END', 'VTIMEZONE')
-			.$event
-			.$this->buildICalLine('END', 'VCALENDAR');
-	}
-
-	protected function buildCalendarDataFragment($item)
-	{
-		$summary = $item->type == 'reservierung' ? $item->titel : $item->topic;
-
-		$description = "";
-		if ($item->type == 'reservierung') {
-			$description = $item->beschreibung;
-		} else {
-			$description = $item->lehrfach_bez . "\r\n";
-
-			if (isset($item->lektor) && is_array($item->lektor) && count($item->lektor) > 0) {
-				$description .= "Lehrer: " . join(", ", array_map(function($teacher) { return $teacher["kurzbz"]; }, $item->lektor)) . "\r\n";
-			}
-		}
-
-		$parsedStartDate = $this->formatICalLocalDateTime($item->isostart);
-		$parsedEndDate = $this->formatICalLocalDateTime($item->isoend);
-		$lastModified = $this->formatICalUtcDateTime(isset($item->updateamum) ? $item->updateamum : null);
-		$dtStamp = $lastModified ?: $this->formatICalUtcDateTime($item->isostart);
-
-		if($dtStamp === null)
-			$dtStamp = '19700101T000000Z';
-
-		$fragment = $this->buildICalLine('BEGIN', 'VEVENT')
-			.$this->buildICalTextLine('UID', $item->eindeutige_gruppen_id)
-			.$this->buildICalLine('SEQUENCE', (int)$item->kalender_id)
-			.$this->buildICalTextLine('SUMMARY', $summary)
-			.$this->buildICalTextLine('DESCRIPTION', 'TODO')
-			.$this->buildICalTextLine('LOCATION', isset($item->ort_kurzbz) ? $item->ort_kurzbz : '')
-			.$this->buildICalTextLine('CATEGORIES', 'Stundenplans')
-			.$this->buildICalLine('DTSTART', $parsedStartDate, array('TZID' => 'Europe/Vienna'))
-			.$this->buildICalLine('DTEND', $parsedEndDate, array('TZID' => 'Europe/Vienna'));
-
-		if($lastModified !== null)
-			$fragment .= $this->buildICalLine('LAST-MODIFIED', $lastModified);
-
-		return $fragment
-			.$this->buildICalLine('DTSTAMP', $dtStamp)
-			.$this->buildICalLine('END', 'VEVENT');
-	}
-
-	protected function buildCalendarCtag($userUID)
-	{
-		$data = $this->getCalendarData($userUID);
-		if(!is_array($data))
-			$data = array();
-
-		$fragments = array();
-		foreach($data as $row)
-		{
-			$lastModified = isset($row->updateamum) ? $this->getLastModifiedTimestamp($row->updateamum) : null;
-			$calendarData = isset($row->calendarDataFragment) ? $this->makeCal($row->calendarDataFragment) : '';
-			$fragments[] = $this->getObjectUri($row).':'.$lastModified.':'.md5($calendarData);
-		}
-
-		sort($fragments, SORT_STRING);
-		return 'LVPlan-'.$userUID.'-'.md5(implode('|', $fragments));
-	}
-
-	protected function buildICalTextLine($name, $value, array $parameters = array())
-	{
-		return $this->buildICalLine($name, $this->escapeICalText($value), $parameters);
-	}
-
-	protected function buildICalLine($name, $value, array $parameters = array())
-	{
-		$line = strtoupper($name);
-
-		foreach($parameters as $parameterName => $parameterValue)
-		{
-			$parameterValues = is_array($parameterValue) ? $parameterValue : array($parameterValue);
-			$escapedValues = array();
-
-			foreach($parameterValues as $singleParameterValue)
-				$escapedValues[] = $this->escapeICalParameterValue($singleParameterValue);
-
-			$line .= ';'.strtoupper($parameterName).'='.implode(',', $escapedValues);
-		}
-
-		return $this->foldICalLine($line.':'.(string)$value);
-	}
-
-	protected function foldICalLine($line)
-	{
-		$line = (string)$line;
-		$folded = '';
-
-		while(strlen($line) > 75)
-		{
-			$part = mb_strcut($line, 0, 75, 'UTF-8');
-			$folded .= $part."\r\n";
-			$line = ' '.mb_strcut($line, strlen($part), strlen($line) - strlen($part), 'UTF-8');
-		}
-
-		return $folded.$line."\r\n";
-	}
-
-	protected function escapeICalText($value)
-	{
-		$value = (string)$value;
-		$value = str_replace('\\', '\\\\', $value);
-		$value = str_replace(array("\r\n", "\r", "\n"), '\n', $value);
-		return str_replace(array(';', ','), array('\;', '\,'), $value);
-	}
-
-	protected function escapeICalParameterValue($value)
-	{
-		$value = (string)$value;
-		$value = str_replace(array('\\', '"', "\r", "\n"), array('\\\\', '\"', '', ''), $value);
-
-		if(preg_match('/[;:,]/', $value))
-			return '"'.$value.'"';
-
-		return $value;
-	}
-
-	protected function formatICalLocalDateTime($dateTime)
-	{
-		$dateTime = $this->createDateTime($dateTime);
-		if($dateTime === null)
-			return null;
-
-		$dateTime->setTimezone(new DateTimeZone('Europe/Vienna'));
-		return $dateTime->format('Ymd\THis');
-	}
-
-	protected function formatICalUtcDateTime($dateTime)
-	{
-		$dateTime = $this->createDateTime($dateTime);
-		if($dateTime === null)
-			return null;
-
-		$dateTime->setTimezone(new DateTimeZone('UTC'));
-		return $dateTime->format('Ymd\THis\Z');
-	}
-
-	protected function createDateTime($dateTime)
-	{
-		if($dateTime instanceof DateTime)
-			return clone $dateTime;
-
-		if($dateTime === null || $dateTime === '')
-			return null;
-
-		try
-		{
-			if(is_numeric($dateTime))
-				return new DateTime('@'.(int)$dateTime);
-
-			return new DateTime($dateTime);
-		}
-		catch(Exception $e)
-		{
-			return null;
-		}
-	}
-
-	protected function getObjectUri($row)
-	{
-		return md5($row->eindeutige_gruppen_id).'.ics';
-	}
-
-	protected function normalizeObjectUri($objectUri)
-	{
-		$normalizedObjectUri = (string)$objectUri;
-
-		if(substr($normalizedObjectUri, -4) === '.ics')
-			$normalizedObjectUri = substr($normalizedObjectUri, 0, -4);
-
-		if(mb_strpos($normalizedObjectUri, '@') !== false)
-			$normalizedObjectUri = mb_substr($normalizedObjectUri, mb_strpos($normalizedObjectUri, '@') + 1);
-
-		if($this->hasObjectUriDatePrefix($normalizedObjectUri))
-			return mb_substr($normalizedObjectUri, mb_strpos($normalizedObjectUri, '-') + 1);
-
-		return $normalizedObjectUri;
-	}
-
-	protected function hasObjectUriDatePrefix($objectUri)
-	{
-		$hyphenPosition = mb_strpos($objectUri, '-');
-		if($hyphenPosition !== 15)
-			return false;
-
-		return preg_match('/^\d{8}T\d{6}$/', mb_substr($objectUri, 0, 15)) === 1;
-	}
-
-	protected function buildCalendarObject($row, $calendarId)
-	{
-		$calendarData = $this->makeCal($row->calendarDataFragment);
-
-		return array(
-			"id"=>$row->eindeutige_gruppen_id,
-			"calendardata"=>$calendarData,
-			"uri"=>$this->getObjectUri($row),
-			"lastmodified"=>$this->getLastModifiedTimestamp($row->updateamum),
-			"etag"=>'"'.md5($calendarData).'"',
-			"calendarid"=>$calendarId,
-			"size"=>strlen($calendarData),
-			"component"=>'vevent'
-		);
-	}
-
-	protected function getLastModifiedTimestamp($lastModified)
-	{
-		if($lastModified === null || $lastModified === '')
-			return null;
-
-		if(is_numeric($lastModified))
-			return (int)$lastModified;
-
-		$timestamp = strtotime($lastModified);
-		if($timestamp === false)
-			return null;
-
-		return $timestamp;
-	}
-
-    /**
-     * Returns all calendar objects within a calendar.
-     *
-     * Every item contains an array with the following keys:
-     *   * id - unique identifier which will be used for subsequent updates
-     *   * calendardata - The iCalendar-compatible calnedar data
-     *   * uri - a unique key which will be used to construct the uri. This can be any arbitrary string.
-     *   * lastmodified - a timestamp of the last modification time
-     *   * etag - An arbitrary string, surrounded by double-quotes. (e.g.:
-     *   '  "abcdef"')
-     *   * calendarid - The calendarid as it was passed to this function.
-     *
-     * Note that the etag is optional, but it's highly encouraged to return for
-     * speed reasons.
-     *
-     * The calendardata is also optional. If it's not returned
-     * 'getCalendarObject' will be called later, which *is* expected to return
-     * calendardata.
-     *
-     * @param string $calendarId
-     * @return array
-     */
-    public function getCalendarObjects($calendarId)
-	{
-		//$user = $this->getUser();
-		$user = $calendarId;
-		$data = $this->getCalendarData($user);
-
-		//error_log("Caldav_Backend.php/getCalendarObjects($calendarId) ");
-		$return  = array();
-
-		if(!is_array($data))
-			return $return;
-
-		foreach($data as $row)
-		{
-			$return[] = $this->buildCalendarObject($row, $calendarId);
-		}
-		return $return;
-    }
-
-    /**
-     * Returns information from a single calendar object, based on it's object
-     * uri.
-     *
-     * The returned array must have the same keys as getCalendarObjects. The
-     * 'calendardata' object is required here though, while it's not required
-     * for getCalendarObjects.
-     *
-     * @param string $calendarId
-     * @param string $objectUri
-     * @return array
-     */
-    public function getCalendarObject($calendarId,$objectUri)
-	{
-		$user = $calendarId;
-		$data = $this->getCalendarData($user,$objectUri);
-
-		if(empty($data))
-		{
-			return null;
-		}
-		elseif(is_object($data))
-		{
-			$ret = $this->buildCalendarObject($data, $calendarId);
-		}
-
-		return isset($ret) ? $ret : null;
-    }
-
-    /**
-     * Creates a new calendar object.
-     *
-     * @param string $calendarId
-     * @param string $objectUri
-     * @param string $calendarData
-     * @return void
-     */
-    public function createCalendarObject($calendarId,$objectUri,$calendarData)
-	{
-		return null;
-    }
-
-    /**
-     * Updates an existing calendarobject, based on it's uri.
-     *
-     * @param string $calendarId
-     * @param string $objectUri
-     * @param string $calendarData
-     * @return void
-     */
-    public function updateCalendarObject($calendarId,$objectUri,$calendarData)
-	{
-		if(is_resource($calendarData))
-			$calendarData = stream_get_contents($calendarData);
-
-		return '"'.md5((string)$calendarData).'"';
-    }
-
-    /**
-     * Deletes an existing calendar object.
-     *
-     * @param string $calendarId
-     * @param string $objectUri
-     * @return void
-     */
-    public function deleteCalendarObject($calendarId,$objectUri)
-	{
-		return null;
-    }
-}
-
-/**
- * Read-only CalDAV node wrappers for DB-owned calendars.
- */
-class SabreDAVReadOnlyACLLib
-{
-	public static function getAcl($principalUri, $includeFreeBusy = false)
-	{
-		$acl = array(
-			array(
-				'privilege' => '{DAV:}read',
-				'principal' => $principalUri,
-				'protected' => true,
-			),
-			array(
-				'privilege' => '{DAV:}read',
-				'principal' => $principalUri.'/calendar-proxy-read',
-				'protected' => true,
-			),
-		);
-
-		if($includeFreeBusy)
-		{
-			$acl[] = array(
-				'privilege' => '{'.\Sabre\CalDAV\Plugin::NS_CALDAV.'}read-free-busy',
-				'principal' => '{DAV:}authenticated',
-				'protected' => true,
-			);
-		}
-
-		return $acl;
-	}
-
-	public static function ignoreWrite()
-	{
-		return null;
-	}
-}
-
-class SabreDAVReadOnlyCalendarRootLib extends \Sabre\CalDAV\CalendarRoot
-{
-	protected $readOnlyCaldavBackend;
-
-	public function __construct(\Sabre\DAVACL\PrincipalBackend\BackendInterface $principalBackend, \Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $principalPrefix = 'principals')
-	{
-		parent::__construct($principalBackend, $caldavBackend, $principalPrefix);
-		$this->readOnlyCaldavBackend = $caldavBackend;
-	}
-
-	public function getChildForPrincipal(array $principal)
-	{
-		return new SabreDAVReadOnlyCalendarHomeLib($this->readOnlyCaldavBackend, $principal);
-	}
-}
-
-class SabreDAVReadOnlyCalendarHomeLib extends \Sabre\CalDAV\CalendarHome
-{
-	protected $readOnlyCaldavBackend;
-	protected $readOnlyPrincipalInfo;
-
-	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $principalInfo)
-	{
-		parent::__construct($caldavBackend, $principalInfo);
-		$this->readOnlyCaldavBackend = $caldavBackend;
-		$this->readOnlyPrincipalInfo = $principalInfo;
-	}
-
-	public function getChildren()
-	{
-		$calendars = $this->readOnlyCaldavBackend->getCalendarsForUser($this->readOnlyPrincipalInfo['uri']);
-		$objs = array();
-
-		foreach($calendars as $calendar)
-		{
-			$objs[] = new SabreDAVReadOnlyCalendarLib($this->readOnlyCaldavBackend, $calendar);
-		}
-
-		return $objs;
-	}
-
-	public function createExtendedCollection($name, \Sabre\DAV\MkCol $mkCol)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function createFile($filename, $data = null)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function createDirectory($filename)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function delete()
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function setName($name)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function getACL()
-	{
-		return SabreDAVReadOnlyACLLib::getAcl($this->readOnlyPrincipalInfo['uri']);
-	}
-}
-
-class SabreDAVReadOnlyCalendarLib extends \Sabre\CalDAV\Calendar
-{
-	protected $readOnlyCaldavBackend;
-	protected $readOnlyCalendarInfo;
-
-	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $calendarInfo)
-	{
-		parent::__construct($caldavBackend, $calendarInfo);
-		$this->readOnlyCaldavBackend = $caldavBackend;
-		$this->readOnlyCalendarInfo = $calendarInfo;
-	}
-
-	public function getChild($name)
-	{
-		$obj = $this->readOnlyCaldavBackend->getCalendarObject($this->readOnlyCalendarInfo['id'], $name);
-
-		if(!$obj)
-			throw new \Sabre\DAV\Exception\NotFound('Calendar object not found');
-
-		$obj['acl'] = $this->getCalendarObjectAcl();
-
-		return new SabreDAVReadOnlyCalendarObjectLib($this->readOnlyCaldavBackend, $this->readOnlyCalendarInfo, $obj);
-	}
-
-	public function getChildren()
-	{
-		$objs = $this->readOnlyCaldavBackend->getCalendarObjects($this->readOnlyCalendarInfo['id']);
-		$children = array();
-
-		foreach($objs as $obj)
-		{
-			$obj['acl'] = $this->getCalendarObjectAcl();
-			$children[] = new SabreDAVReadOnlyCalendarObjectLib($this->readOnlyCaldavBackend, $this->readOnlyCalendarInfo, $obj);
-		}
-
-		return $children;
-	}
-
-	public function createFile($name, $calendarData = null)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function createDirectory($name)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function delete()
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function setName($newName)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function updateProperties($mutations)
-	{
-		return true;
-	}
-
-	public function propPatch(\Sabre\DAV\PropPatch $propPatch)
-	{
-		$propPatch->setRemainingResultCode(200);
-	}
-
-	public function getACL()
-	{
-		return SabreDAVReadOnlyACLLib::getAcl($this->getOwner(), true);
-	}
-
-	protected function getCalendarObjectAcl()
-	{
-		return SabreDAVReadOnlyACLLib::getAcl($this->getOwner());
-	}
-}
-
-class SabreDAVReadOnlyCalendarObjectLib extends \Sabre\CalDAV\CalendarObject
-{
-	protected $readOnlyCalendarInfo;
-
-	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, array $calendarInfo, array $objectData)
-	{
-		parent::__construct($caldavBackend, $calendarInfo, $objectData);
-		$this->readOnlyCalendarInfo = $calendarInfo;
-	}
-
-	public function put($calendarData)
-	{
-		return $this->getETag();
-	}
-
-	public function getETag()
-	{
-		if(isset($_SERVER['REQUEST_METHOD']) && in_array($_SERVER['REQUEST_METHOD'], array('PUT', 'DELETE'), true))
-		{
-			$ifMatch = isset($_SERVER['HTTP_IF_MATCH']) ? trim($_SERVER['HTTP_IF_MATCH']) : '';
-			if($ifMatch !== '' && $ifMatch !== '*')
-			{
-				$requestedEtags = explode(',', $ifMatch);
-				foreach($requestedEtags as $requestedEtag)
-				{
-					$requestedEtag = trim($requestedEtag);
-					if($requestedEtag !== '')
-						return $requestedEtag;
-				}
-			}
-		}
-
-		return parent::getETag();
-	}
-
-	public function delete()
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function setName($name)
-	{
-		return SabreDAVReadOnlyACLLib::ignoreWrite();
-	}
-
-	public function getACL()
-	{
-		return SabreDAVReadOnlyACLLib::getAcl($this->readOnlyCalendarInfo['principaluri']);
-	}
-}
diff --git a/application/libraries/sabredav/caldav/SabreDAVACLPrincipalBackend.php b/application/libraries/sabredav/caldav/SabreDAVACLPrincipalBackend.php
new file mode 100644
index 000000000..822955c8d
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVACLPrincipalBackend.php
@@ -0,0 +1,179 @@
+<?php
+
+/**
+ * PDO principal backend
+ *
+ * This is a simple principal backend that maps exactly to the users table, as
+ * used by Sabre_DAV_Auth_Backend_PDO.
+ *
+ * It assumes all principals are in a single collection. The default collection
+ * is 'principals/', but this can be overriden.
+ *
+ * @package Sabre
+ * @subpackage DAVACL
+ * @copyright Copyright (C) 2007-2011 Rooftop Solutions. All rights reserved.
+ * @author Evert Pot (http://www.rooftopsolutions.nl/)
+ * @license http://code.google.com/p/sabredav/wiki/License Modified BSD License
+ */
+class SabreDAVACLPrincipalBackend implements \Sabre\DAVACL\PrincipalBackend\BackendInterface
+{
+    /**
+     * PDO table name for 'principals'
+     *
+     * @var string
+     */
+    protected $tableName;
+
+    /**
+     * PDO table name for 'group members'
+     *
+     * @var string
+     */
+    protected $groupMembersTableName;
+
+	protected $result_ma;
+	protected $auth;
+    /**
+     * Sets up the backend.
+     *
+     * @param PDO $pdo
+     * @param string $tableName
+     */
+    public function __construct($auth)
+	{
+		$this->auth = $auth;
+
+		/*
+		$ma = new mitarbeiter();
+		$this->result_ma = $ma->getMitarbeiter(null,null,null);
+		*/
+    }
+
+	/**
+	 * Liefert den eingeloggten User
+	 */
+	function getUser()
+	{
+		return $this->auth->getCurrentUser();
+	}
+
+
+    /**
+     * Returns a list of principals based on a prefix.
+     *
+     * This prefix will often contain something like 'principals'. You are only
+     * expected to return principals that are in this base path.
+     *
+     * You are expected to return at least a 'uri' for every user, you can
+     * return any additional properties if you wish so. Common properties are:
+     *   {DAV:}displayname
+     *   {http://sabredav.org/ns}email-address - This is a custom SabreDAV
+     *     field that's actualy injected in a number of other properties. If
+     *     you have an email address, use this property.
+     *
+     * @param string $prefixPath
+     * @return array
+     */
+    public function getPrincipalsByPrefix($prefixPath)
+	{
+		//$prefixPath = principals
+		//error_log('Principal.php/getPrincipalsByPrefix('.$prefixPath.')');
+        $principals = array();
+		$user = $this->getUser();
+
+		if($prefixPath=='principals')
+		{
+
+		    $principals[] = array(
+					'id' => $user,
+		            'uri' => 'principals/'.$user,
+		            '{DAV:}displayname' => $user,
+		            '{http://sabredav.org/ns}email-address' => $user.'@example.com',
+		        );
+		}
+
+        return $principals;
+
+    }
+
+    /**
+     * Returns a specific principal, specified by it's path.
+     * The returned structure should be the exact same as from
+     * getPrincipalsByPrefix.
+     *
+     * @param string $path
+     * @return array
+     */
+    public function getPrincipalByPath($path)
+	{
+		$user = mb_substr($path,11);
+
+        $result = array(
+            'id'  => $user,
+            'uri' => 'principals/'.$user,
+            '{DAV:}displayname' => $user,
+            '{http://sabredav.org/ns}email-address' => $user.'@example.com',
+        );
+
+		return $result;
+    }
+
+    /**
+     * Returns the list of members for a group-principal
+     *
+     * @param string $principal
+     * @return array
+     */
+    public function getGroupMemberSet($principal)
+	{
+        $result = array();
+
+		return $result;
+    }
+
+    /**
+     * Returns the list of groups a principal is a member of
+     *
+     * @param string $principal
+     * @return array
+     */
+    public function getGroupMembership($principal)
+	{
+		$result = array();
+		if(preg_match('/^principals\/[0-9A-Za-z\-]*$/',$principal))
+		{
+			$user = mb_substr($principal,11);
+		}
+        return $result;
+
+    }
+
+    /**
+     * Updates the list of group members for a group principal.
+     *
+     * The principals should be passed as a list of uri's.
+     *
+     * @param string $principal
+     * @param array $members
+     * @return void
+     */
+    public function setGroupMemberSet($principal, array $members)
+	{
+		throw new \Sabre\DAV\Exception('Not implemented');
+    }
+
+   public function updatePrincipal($path, \Sabre\DAV\PropPatch $propPatch)
+   {
+	$propPatch->setRemainingResultCode(403);
+   }
+
+	public function searchPrincipals($prefixPath, array $searchProperties, $test = 'allof')
+	{
+		return array();
+	}
+
+	public function findByUri($uri, $principalPrefix)
+	{
+		return null;
+	}
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVCalDAVBackend.php b/application/libraries/sabredav/caldav/SabreDAVCalDAVBackend.php
new file mode 100644
index 000000000..2bfa12804
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVCalDAVBackend.php
@@ -0,0 +1,546 @@
+<?php
+
+/**
+ * CalDAV backend
+ */
+class SabreDAVCalDAVBackend extends \Sabre\CalDAV\Backend\AbstractBackend
+{
+	protected $auth;
+
+	protected $CI;
+
+	/**
+     * Creates the backend
+     *
+     * @param AuthBackend $auth
+     */
+    public function __construct($auth)
+	{
+		$this->auth = $auth;
+
+		$this->CI =& get_instance();
+    }
+
+	/**
+	 * Liefert den eingeloggten User
+	 */
+	function getUser()
+	{
+		return $this->auth->getCurrentUser();
+	}
+
+    /**
+     * Returns a list of calendars for a principal.
+     *
+     * Every project is an array with the following keys:
+     *  * id, a unique id that will be used by other functions to modify the
+     *    calendar. This can be the same as the uri or a database key.
+     *  * uri, which the basename of the uri with which the calendar is
+     *    accessed.
+     *  * principalUri. The owner of the calendar. Almost always the same as
+     *    principalUri passed to this method.
+     *
+     * Furthermore it can contain webdav properties in clark notation. A very
+     * common one is '{DAV:}displayname'.
+     *
+     * @param string $principalUri
+     * @return array
+     */
+    public function getCalendarsForUser($principalUri)
+	{
+		$user = mb_substr($principalUri,11);
+        $calendars = array();
+		$calendar = array(
+			'id' => $user,
+			'uri' => 'LVPlan-'.$user,
+			'principaluri' => 'principals/'.$user,
+			'{' . \Sabre\CalDAV\Plugin::NS_CALENDARSERVER . '}getctag' => $this->buildCalendarCtag($user),
+			'{' . \Sabre\CalDAV\Plugin::NS_CALDAV . '}supported-calendar-component-set' => new \Sabre\CalDAV\Xml\Property\SupportedCalendarComponentSet(array('VEVENT')),
+			'{DAV:}displayname'                          => 'LVPlan',
+			'{urn:ietf:params:xml:ns:caldav}calendar-description' => 'description comes here',
+			'{urn:ietf:params:xml:ns:caldav}calendar-timezone'    => 'Europe/Vienna',
+			'{http://apple.com/ns/ical/}calendar-order'  => '1',
+			'{http://apple.com/ns/ical/}calendar-color'  => '#FF0000',
+			'{http://sabredav.org/ns}read-only' => 1
+        );
+		$calendars[] = $calendar;
+
+        return $calendars;
+    }
+
+    /**
+     * Creates a new calendar for a principal.
+     *
+     * If the creation was a success, an id must be returned that can be used to reference
+     * this calendar in other methods, such as updateCalendar
+     *
+     * @param string $principalUri
+     * @param string $calendarUri
+     * @param array $properties
+     */
+    public function createCalendar($principalUri,$calendarUri, array $properties)
+	{
+		return $calendarUri;
+    }
+
+    /**
+     * Updates properties for a calendar.
+     *
+     * The mutations array uses the propertyName in clark-notation as key,
+     * and the array value for the property value. In the case a property
+     * should be deleted, the property value will be null.
+     *
+     * This method must be atomic. If one property cannot be changed, the
+     * entire operation must fail.
+     *
+     * If the operation was successful, true can be returned.
+     * If the operation failed, false can be returned.
+     *
+     * Deletion of a non-existant property is always succesful.
+     *
+     * Lastly, it is optional to return detailed information about any
+     * failures. In this case an array should be returned with the following
+     * structure:
+     *
+     * array(
+     *   403 => array(
+     *      '{DAV:}displayname' => null,
+     *   ),
+     *   424 => array(
+     *      '{DAV:}owner' => null,
+     *   )
+     * )
+     *
+     * In this example it was forbidden to update {DAV:}displayname.
+     * (403 Forbidden), which in turn also caused {DAV:}owner to fail
+     * (424 Failed Dependency) because the request needs to be atomic.
+     *
+     * @param string $calendarId
+     * @param array $mutations
+     * @return bool|array
+     */
+    public function updateCalendar($calendarId, \Sabre\DAV\PropPatch $propPatch)
+	{
+        $propPatch->setRemainingResultCode(200);
+    }
+
+    /**
+     * Delete a calendar and all it's objects
+     *
+     * @param string $calendarId
+     * @return void
+     */
+    public function deleteCalendar($calendarId)
+	{
+		return null;
+    }
+
+
+	public function getCalendarData($userUID, $objectUri=null)
+	{
+		$this->CI->load->model("ressource/Mitarbeiter_model", "MitarbeiterModel");
+		$this->CI->load->model("person/Benutzer_model", "BenutzerModel");
+
+		$user = $this->CI->BenutzerModel->loadWhere(array("uid" => $userUID));
+		if(!$user)
+			die('User invalid');
+
+		$this->CI->load->library('KalenderLib', ['uid' => $userUID]);
+
+		$isUserEmployeeResult = $this->CI->MitarbeiterModel->isMitarbeiter($userUID);
+		if (isError($isUserEmployeeResult)) {
+			return;
+		}
+		$isUserEmployee = getData($isUserEmployeeResult);
+
+		$startDate = date('Y-m-d', mktime(0,0,0,date('m'),date('d')-14,date('Y')));
+		$endDate = date('Y-m-d', mktime(0,0,0,date('m')+6,date('d'),date('Y')));
+
+		$data = [];
+
+		if ($isUserEmployee) {
+			$data = $this->CI->kalenderlib->getPlanForLecturerByLecturer($startDate, $endDate, $userUID);
+		} else {
+			$data = $this->CI->kalenderlib->getPlanForStudentByStudent($startDate, $endDate, $userUID);
+		}
+
+		if(!is_array($data))
+			$data = array();
+
+		foreach($data as $item)
+		{
+			$item->calendarDataFragment = $this->buildCalendarDataFragment($item);
+		}
+
+		if (is_null($objectUri))
+		{
+			return $data;
+		}
+
+		$normalizedObjectUri = $this->normalizeObjectUri($objectUri);
+		foreach($data as $row)
+		{
+			if(md5($row->eindeutige_gruppen_id) == $normalizedObjectUri)
+			{
+				return $row;
+			}
+		}
+
+		return null;
+	}
+
+	public function makeCal($event)
+	{
+		return $this->buildICalLine('BEGIN', 'VCALENDAR')
+			.$this->buildICalLine('VERSION', '2.0')
+			.$this->buildICalTextLine('PRODID', 'FH Technikum Wien')
+			.$this->buildICalLine('BEGIN', 'VTIMEZONE')
+			.$this->buildICalLine('TZID', 'Europe/Vienna')
+			.$this->buildICalLine('BEGIN', 'DAYLIGHT')
+			.$this->buildICalLine('TZOFFSETFROM', '+0100')
+			.$this->buildICalLine('RRULE', 'FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU')
+			.$this->buildICalLine('DTSTART', '19810329T020000')
+			.$this->buildICalLine('TZNAME', 'GMT+02:00')
+			.$this->buildICalLine('TZOFFSETTO', '+0200')
+			.$this->buildICalLine('END', 'DAYLIGHT')
+			.$this->buildICalLine('BEGIN', 'STANDARD')
+			.$this->buildICalLine('TZOFFSETFROM', '+0200')
+			.$this->buildICalLine('RRULE', 'FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU')
+			.$this->buildICalLine('DTSTART', '19961027T030000')
+			.$this->buildICalLine('TZNAME', 'GMT+01:00')
+			.$this->buildICalLine('TZOFFSETTO', '+0100')
+			.$this->buildICalLine('END', 'STANDARD')
+			.$this->buildICalLine('END', 'VTIMEZONE')
+			.$event
+			.$this->buildICalLine('END', 'VCALENDAR');
+	}
+
+	protected function buildCalendarDataFragment($item)
+	{
+		$summary = $item->type == 'reservierung' ? $item->titel : $item->topic;
+
+		$description = "";
+		if ($item->type == 'reservierung') {
+			$description = $item->beschreibung;
+		} else {
+			$description = $item->lehrfach_bez . "\r\n";
+
+			if (isset($item->lektor) && is_array($item->lektor) && count($item->lektor) > 0) {
+				$description .= "Lehrer: " . join(", ", array_map(function($teacher) { return $teacher["kurzbz"]; }, $item->lektor)) . "\r\n";
+			}
+		}
+
+		$parsedStartDate = $this->formatICalLocalDateTime($item->isostart);
+		$parsedEndDate = $this->formatICalLocalDateTime($item->isoend);
+		$lastModified = $this->formatICalUtcDateTime(isset($item->updateamum) ? $item->updateamum : null);
+		$dtStamp = $lastModified ?: $this->formatICalUtcDateTime($item->isostart);
+
+		if($dtStamp === null)
+			$dtStamp = '19700101T000000Z';
+
+		$fragment = $this->buildICalLine('BEGIN', 'VEVENT')
+			.$this->buildICalTextLine('UID', $item->eindeutige_gruppen_id)
+			.$this->buildICalLine('SEQUENCE', (int)$item->kalender_id)
+			.$this->buildICalTextLine('SUMMARY', $summary)
+			.$this->buildICalTextLine('DESCRIPTION', $description)
+			.$this->buildICalTextLine('LOCATION', isset($item->ort_kurzbz) ? $item->ort_kurzbz : '')
+			.$this->buildICalTextLine('CATEGORIES', 'Stundenplans')
+			.$this->buildICalLine('DTSTART', $parsedStartDate, array('TZID' => 'Europe/Vienna'))
+			.$this->buildICalLine('DTEND', $parsedEndDate, array('TZID' => 'Europe/Vienna'));
+
+		if($lastModified !== null)
+			$fragment .= $this->buildICalLine('LAST-MODIFIED', $lastModified);
+
+		return $fragment
+			.$this->buildICalLine('DTSTAMP', $dtStamp)
+			.$this->buildICalLine('END', 'VEVENT');
+	}
+
+	protected function buildCalendarCtag($userUID)
+	{
+		$data = $this->getCalendarData($userUID);
+		if(!is_array($data))
+			$data = array();
+
+		$fragments = array();
+		foreach($data as $row)
+		{
+			$lastModified = isset($row->updateamum) ? $this->getLastModifiedTimestamp($row->updateamum) : null;
+			$calendarData = isset($row->calendarDataFragment) ? $this->makeCal($row->calendarDataFragment) : '';
+			$fragments[] = $this->getObjectUri($row).':'.$lastModified.':'.md5($calendarData);
+		}
+
+		sort($fragments, SORT_STRING);
+		return 'LVPlan-'.$userUID.'-'.md5(implode('|', $fragments));
+	}
+
+	protected function buildICalTextLine($name, $value, array $parameters = array())
+	{
+		return $this->buildICalLine($name, $this->escapeICalText($value), $parameters);
+	}
+
+	protected function buildICalLine($name, $value, array $parameters = array())
+	{
+		$line = strtoupper($name);
+
+		foreach($parameters as $parameterName => $parameterValue)
+		{
+			$parameterValues = is_array($parameterValue) ? $parameterValue : array($parameterValue);
+			$escapedValues = array();
+
+			foreach($parameterValues as $singleParameterValue)
+				$escapedValues[] = $this->escapeICalParameterValue($singleParameterValue);
+
+			$line .= ';'.strtoupper($parameterName).'='.implode(',', $escapedValues);
+		}
+
+		return $this->foldICalLine($line.':'.(string)$value);
+	}
+
+	protected function foldICalLine($line)
+	{
+		$line = (string)$line;
+		$folded = '';
+
+		while(strlen($line) > 75)
+		{
+			$part = mb_strcut($line, 0, 75, 'UTF-8');
+			$folded .= $part."\r\n";
+			$line = ' '.mb_strcut($line, strlen($part), strlen($line) - strlen($part), 'UTF-8');
+		}
+
+		return $folded.$line."\r\n";
+	}
+
+	protected function escapeICalText($value)
+	{
+		$value = (string)$value;
+		$value = str_replace('\\', '\\\\', $value);
+		$value = str_replace(array("\r\n", "\r", "\n"), '\n', $value);
+		return str_replace(array(';', ','), array('\;', '\,'), $value);
+	}
+
+	protected function escapeICalParameterValue($value)
+	{
+		$value = (string)$value;
+		$value = str_replace(array('\\', '"', "\r", "\n"), array('\\\\', '\"', '', ''), $value);
+
+		if(preg_match('/[;:,]/', $value))
+			return '"'.$value.'"';
+
+		return $value;
+	}
+
+	protected function formatICalLocalDateTime($dateTime)
+	{
+		$dateTime = $this->createDateTime($dateTime);
+		if($dateTime === null)
+			return null;
+
+		$dateTime->setTimezone(new DateTimeZone('Europe/Vienna'));
+		return $dateTime->format('Ymd\THis');
+	}
+
+	protected function formatICalUtcDateTime($dateTime)
+	{
+		$dateTime = $this->createDateTime($dateTime);
+		if($dateTime === null)
+			return null;
+
+		$dateTime->setTimezone(new DateTimeZone('UTC'));
+		return $dateTime->format('Ymd\THis\Z');
+	}
+
+	protected function createDateTime($dateTime)
+	{
+		if($dateTime instanceof DateTime)
+			return clone $dateTime;
+
+		if($dateTime === null || $dateTime === '')
+			return null;
+
+		try
+		{
+			if(is_numeric($dateTime))
+				return new DateTime('@'.(int)$dateTime);
+
+			return new DateTime($dateTime);
+		}
+		catch(Exception $e)
+		{
+			return null;
+		}
+	}
+
+	protected function getObjectUri($row)
+	{
+		return md5($row->eindeutige_gruppen_id).'.ics';
+	}
+
+	protected function normalizeObjectUri($objectUri)
+	{
+		$normalizedObjectUri = (string)$objectUri;
+
+		if(substr($normalizedObjectUri, -4) === '.ics')
+			$normalizedObjectUri = substr($normalizedObjectUri, 0, -4);
+
+		if(mb_strpos($normalizedObjectUri, '@') !== false)
+			$normalizedObjectUri = mb_substr($normalizedObjectUri, mb_strpos($normalizedObjectUri, '@') + 1);
+
+		if($this->hasObjectUriDatePrefix($normalizedObjectUri))
+			return mb_substr($normalizedObjectUri, mb_strpos($normalizedObjectUri, '-') + 1);
+
+		return $normalizedObjectUri;
+	}
+
+	protected function hasObjectUriDatePrefix($objectUri)
+	{
+		$hyphenPosition = mb_strpos($objectUri, '-');
+		if($hyphenPosition !== 15)
+			return false;
+
+		return preg_match('/^\d{8}T\d{6}$/', mb_substr($objectUri, 0, 15)) === 1;
+	}
+
+	protected function buildCalendarObject($row, $calendarId)
+	{
+		$calendarData = $this->makeCal($row->calendarDataFragment);
+
+		return array(
+			"id"=>$row->eindeutige_gruppen_id,
+			"calendardata"=>$calendarData,
+			"uri"=>$this->getObjectUri($row),
+			"lastmodified"=>$this->getLastModifiedTimestamp($row->updateamum),
+			"etag"=>'"'.md5($calendarData).'"',
+			"calendarid"=>$calendarId,
+			"size"=>strlen($calendarData),
+			"component"=>'vevent'
+		);
+	}
+
+	protected function getLastModifiedTimestamp($lastModified)
+	{
+		if($lastModified === null || $lastModified === '')
+			return null;
+
+		if(is_numeric($lastModified))
+			return (int)$lastModified;
+
+		$timestamp = strtotime($lastModified);
+		if($timestamp === false)
+			return null;
+
+		return $timestamp;
+	}
+
+    /**
+     * Returns all calendar objects within a calendar.
+     *
+     * Every item contains an array with the following keys:
+     *   * id - unique identifier which will be used for subsequent updates
+     *   * calendardata - The iCalendar-compatible calnedar data
+     *   * uri - a unique key which will be used to construct the uri. This can be any arbitrary string.
+     *   * lastmodified - a timestamp of the last modification time
+     *   * etag - An arbitrary string, surrounded by double-quotes. (e.g.:
+     *   '  "abcdef"')
+     *   * calendarid - The calendarid as it was passed to this function.
+     *
+     * Note that the etag is optional, but it's highly encouraged to return for
+     * speed reasons.
+     *
+     * The calendardata is also optional. If it's not returned
+     * 'getCalendarObject' will be called later, which *is* expected to return
+     * calendardata.
+     *
+     * @param string $calendarId
+     * @return array
+     */
+    public function getCalendarObjects($calendarId)
+	{
+		//$user = $this->getUser();
+		$user = $calendarId;
+		$data = $this->getCalendarData($user);
+
+		//error_log("Caldav_Backend.php/getCalendarObjects($calendarId) ");
+		$return  = array();
+
+		if(!is_array($data))
+			return $return;
+
+		foreach($data as $row)
+		{
+			$return[] = $this->buildCalendarObject($row, $calendarId);
+		}
+		return $return;
+    }
+
+    /**
+     * Returns information from a single calendar object, based on it's object
+     * uri.
+     *
+     * The returned array must have the same keys as getCalendarObjects. The
+     * 'calendardata' object is required here though, while it's not required
+     * for getCalendarObjects.
+     *
+     * @param string $calendarId
+     * @param string $objectUri
+     * @return array
+     */
+    public function getCalendarObject($calendarId,$objectUri)
+	{
+		$user = $calendarId;
+		$data = $this->getCalendarData($user,$objectUri);
+
+		if(empty($data))
+		{
+			return null;
+		}
+		elseif(is_object($data))
+		{
+			$ret = $this->buildCalendarObject($data, $calendarId);
+		}
+
+		return isset($ret) ? $ret : null;
+    }
+
+    /**
+     * Creates a new calendar object.
+     *
+     * @param string $calendarId
+     * @param string $objectUri
+     * @param string $calendarData
+     * @return void
+     */
+    public function createCalendarObject($calendarId,$objectUri,$calendarData)
+	{
+		return null;
+    }
+
+    /**
+     * Updates an existing calendarobject, based on it's uri.
+     *
+     * @param string $calendarId
+     * @param string $objectUri
+     * @param string $calendarData
+     * @return void
+     */
+    public function updateCalendarObject($calendarId,$objectUri,$calendarData)
+	{
+		if(is_resource($calendarData))
+			$calendarData = stream_get_contents($calendarData);
+
+		return '"'.md5((string)$calendarData).'"';
+    }
+
+    /**
+     * Deletes an existing calendar object.
+     *
+     * @param string $calendarId
+     * @param string $objectUri
+     * @return void
+     */
+    public function deleteCalendarObject($calendarId,$objectUri)
+	{
+		return null;
+    }
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVReadOnlyACL.php b/application/libraries/sabredav/caldav/SabreDAVReadOnlyACL.php
new file mode 100644
index 000000000..5407631d5
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVReadOnlyACL.php
@@ -0,0 +1,36 @@
+<?php
+
+class SabreDAVReadOnlyACL
+{
+	public static function getAcl($principalUri, $includeFreeBusy = false)
+	{
+		$acl = array(
+			array(
+				'privilege' => '{DAV:}read',
+				'principal' => $principalUri,
+				'protected' => true,
+			),
+			array(
+				'privilege' => '{DAV:}read',
+				'principal' => $principalUri.'/calendar-proxy-read',
+				'protected' => true,
+			),
+		);
+
+		if($includeFreeBusy)
+		{
+			$acl[] = array(
+				'privilege' => '{'.\Sabre\CalDAV\Plugin::NS_CALDAV.'}read-free-busy',
+				'principal' => '{DAV:}authenticated',
+				'protected' => true,
+			);
+		}
+
+		return $acl;
+	}
+
+	public static function ignoreWrite()
+	{
+		return null;
+	}
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendar.php b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendar.php
new file mode 100644
index 000000000..37f5ff8ae
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendar.php
@@ -0,0 +1,80 @@
+<?php
+
+class SabreDAVReadOnlyCalendar extends \Sabre\CalDAV\Calendar
+{
+	protected $readOnlyCaldavBackend;
+	protected $readOnlyCalendarInfo;
+
+	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $calendarInfo)
+	{
+		parent::__construct($caldavBackend, $calendarInfo);
+		$this->readOnlyCaldavBackend = $caldavBackend;
+		$this->readOnlyCalendarInfo = $calendarInfo;
+	}
+
+	public function getChild($name)
+	{
+		$obj = $this->readOnlyCaldavBackend->getCalendarObject($this->readOnlyCalendarInfo['id'], $name);
+
+		if(!$obj)
+			throw new \Sabre\DAV\Exception\NotFound('Calendar object not found');
+
+		$obj['acl'] = $this->getCalendarObjectAcl();
+
+		return new SabreDAVReadOnlyCalendarObject($this->readOnlyCaldavBackend, $this->readOnlyCalendarInfo, $obj);
+	}
+
+	public function getChildren()
+	{
+		$objs = $this->readOnlyCaldavBackend->getCalendarObjects($this->readOnlyCalendarInfo['id']);
+		$children = array();
+
+		foreach($objs as $obj)
+		{
+			$obj['acl'] = $this->getCalendarObjectAcl();
+			$children[] = new SabreDAVReadOnlyCalendarObject($this->readOnlyCaldavBackend, $this->readOnlyCalendarInfo, $obj);
+		}
+
+		return $children;
+	}
+
+	public function createFile($name, $calendarData = null)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function createDirectory($name)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function delete()
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function setName($newName)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function updateProperties($mutations)
+	{
+		return true;
+	}
+
+	public function propPatch(\Sabre\DAV\PropPatch $propPatch)
+	{
+		$propPatch->setRemainingResultCode(200);
+	}
+
+	public function getACL()
+	{
+		return SabreDAVReadOnlyACL::getAcl($this->getOwner(), true);
+	}
+
+	protected function getCalendarObjectAcl()
+	{
+		return SabreDAVReadOnlyACL::getAcl($this->getOwner());
+	}
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarHome.php b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarHome.php
new file mode 100644
index 000000000..2c0933b5b
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarHome.php
@@ -0,0 +1,57 @@
+<?php
+
+class SabreDAVReadOnlyCalendarHome extends \Sabre\CalDAV\CalendarHome
+{
+	protected $readOnlyCaldavBackend;
+	protected $readOnlyPrincipalInfo;
+
+	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $principalInfo)
+	{
+		parent::__construct($caldavBackend, $principalInfo);
+		$this->readOnlyCaldavBackend = $caldavBackend;
+		$this->readOnlyPrincipalInfo = $principalInfo;
+	}
+
+	public function getChildren()
+	{
+		$calendars = $this->readOnlyCaldavBackend->getCalendarsForUser($this->readOnlyPrincipalInfo['uri']);
+		$objs = array();
+
+		foreach($calendars as $calendar)
+		{
+			$objs[] = new SabreDAVReadOnlyCalendar($this->readOnlyCaldavBackend, $calendar);
+		}
+
+		return $objs;
+	}
+
+	public function createExtendedCollection($name, \Sabre\DAV\MkCol $mkCol)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function createFile($filename, $data = null)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function createDirectory($filename)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function delete()
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function setName($name)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function getACL()
+	{
+		return SabreDAVReadOnlyACL::getAcl($this->readOnlyPrincipalInfo['uri']);
+	}
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarObject.php b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarObject.php
new file mode 100644
index 000000000..4b15c20c3
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarObject.php
@@ -0,0 +1,84 @@
+<?php
+
+class SabreDAVReadOnlyCalendarObject extends \Sabre\CalDAV\CalendarObject
+{
+	protected $readOnlyCalendarInfo;
+
+	public function __construct(\Sabre\CalDAV\Backend\BackendInterface $caldavBackend, array $calendarInfo, array $objectData)
+	{
+		parent::__construct($caldavBackend, $calendarInfo, $objectData);
+		$this->readOnlyCalendarInfo = $calendarInfo;
+	}
+
+	public function put($calendarData)
+	{
+		return $this->getCurrentETag();
+	}
+
+	public function getETag()
+	{
+		if($this->isWriteRequest())
+		{
+			$ifMatch = isset($_SERVER['HTTP_IF_MATCH']) ? trim($_SERVER['HTTP_IF_MATCH']) : '';
+			if($ifMatch !== '' && $ifMatch !== '*')
+			{
+				$requestedEtags = explode(',', $ifMatch);
+				foreach($requestedEtags as $requestedEtag)
+				{
+					$requestedEtag = trim($requestedEtag);
+					if($requestedEtag !== '')
+						return $requestedEtag;
+				}
+			}
+		}
+
+		return parent::getETag();
+	}
+
+	public function getLastModified()
+	{
+		if($this->isWriteRequest())
+		{
+			$ifUnmodifiedSince = isset($_SERVER['HTTP_IF_UNMODIFIED_SINCE']) ? trim($_SERVER['HTTP_IF_UNMODIFIED_SINCE']) : '';
+			if($ifUnmodifiedSince !== '')
+			{
+				$timestamp = strtotime($ifUnmodifiedSince);
+				if($timestamp !== false)
+					return max(0, $timestamp - 1);
+			}
+		}
+
+		return parent::getLastModified();
+	}
+
+	public function delete()
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function setName($name)
+	{
+		return SabreDAVReadOnlyACL::ignoreWrite();
+	}
+
+	public function getACL()
+	{
+		return SabreDAVReadOnlyACL::getAcl($this->readOnlyCalendarInfo['principaluri']);
+	}
+
+	protected function isWriteRequest()
+	{
+		if(!isset($_SERVER['REQUEST_METHOD']))
+			return false;
+
+		return in_array(strtoupper($_SERVER['REQUEST_METHOD']), array('PUT', 'DELETE', 'PROPPATCH', 'MOVE', 'COPY', 'PATCH'), true);
+	}
+
+	protected function getCurrentETag()
+	{
+		if(isset($this->objectData['etag']) && $this->objectData['etag'] !== '')
+			return $this->objectData['etag'];
+
+		return parent::getETag();
+	}
+}
diff --git a/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarRoot.php b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarRoot.php
new file mode 100644
index 000000000..345c30b12
--- /dev/null
+++ b/application/libraries/sabredav/caldav/SabreDAVReadOnlyCalendarRoot.php
@@ -0,0 +1,17 @@
+<?php
+
+class SabreDAVReadOnlyCalendarRoot extends \Sabre\CalDAV\CalendarRoot
+{
+	protected $readOnlyCaldavBackend;
+
+	public function __construct(\Sabre\DAVACL\PrincipalBackend\BackendInterface $principalBackend, \Sabre\CalDAV\Backend\BackendInterface $caldavBackend, $principalPrefix = 'principals')
+	{
+		parent::__construct($principalBackend, $caldavBackend, $principalPrefix);
+		$this->readOnlyCaldavBackend = $caldavBackend;
+	}
+
+	public function getChildForPrincipal(array $principal)
+	{
+		return new SabreDAVReadOnlyCalendarHome($this->readOnlyCaldavBackend, $principal);
+	}
+}