nextcloud/apps/calendar/ajax/settings/guesstimezone.php

<?php
/**
 * Copyright (c) 2011, 2012 Georg Ehrke <ownclouddev at georgswebsite dot de>
 * This file is licensed under the Affero General Public License version 3 or
 * later.
 * See the COPYING-README file.
 */
 

OCP\JSON::checkLoggedIn();
OCP\JSON::checkAppEnabled('calendar');

$l = OC_L10N::get('calendar');

$lat = $_POST['lat'];
$lng = $_POST['lng'];

$timezone =  OC_Geo::timezone($lat, $lng);

if($timezone == OCP\Config::getUserValue(OCP\USER::getUser(), 'calendar', 'timezone')){
	OCP\JSON::success();
	exit;
}
OCP\Config::setUserValue(OCP\USER::getUser(), 'calendar', 'timezone', $timezone);
$message = array('message'=> $l->t('New Timezone:') . $timezone);
OCP\JSON::success($message);