src/YAAP/Theme/Theme.php
private function _assetVersion($path)
    {

        $full_path = $this->_assetFullpath($path);

        $file_path = public_path($full_path);

        if (file_exists($file_path)) {
            return filemtime($file_path);
        }

        return null;
    }
private function _assetVersion($path)
    {

        $full_path = $this->_assetFullpath($path);

        $file_path = public_path($full_path);

        if (file_exists($file_path)) {
            return filemtime($file_path);
        }

        return null;
    }
Return filemtime of given asset or null if asset doesn't exists

@param $path
@return int|null