public static function lastModifiedFolder($path)
    {
        $last_modified = 0;

        /** @var UniformResourceLocator $locator */
        $locator = Grav::instance()['locator'];
        $flags = \RecursiveDirectoryIterator::SKIP_DOTS;
        if ($locator->isStream($path)) {
            $directory = $locator->getRecursiveIterator($path, $flags);
        } else {
            $directory = new \RecursiveDirectoryIterator($path, $flags);
        }
        $filter  = new RecursiveFolderFilterIterator($directory);
        $iterator = new \RecursiveIteratorIterator($filter, \RecursiveIteratorIterator::SELF_FIRST);

        /** @var \RecursiveDirectoryIterator $file */
        foreach ($iterator as $dir) {
            $dir_modified = $dir->getMTime();
            if ($dir_modified > $last_modified) {
                $last_modified = $dir_modified;
            }
        }

        return $last_modified;
    }