public function getLastTimeModified()
{
if ($this->_lastTimeModified === null) {
$this->_lastTimeModified = IOHelper::getLastTimeModified($this->filenamePath);
if ($this->_lastTimeModified === false) {
throw new Exception('Minimee could not determine modification time of local asset: ' . $this->filenamePath);
}
}
return $this->_lastTimeModified;
}