code: protected function getStorageKey($key, $limit, $milliseconds)
      {
          $window = $milliseconds * (floor((microtime(1) * 1000)/$milliseconds));
          $date = date('YmdHis', $window/1000);
          return $date . '::' . $key . '::' . $limit . '::' . $milliseconds . '::COUNT'; 
      }
docs: Get storage key
  
  @param string $key  - A unique key for what we're throttling
  @param int $limit   - How many are allowed
  @param int $milliseconds - In this many milliseconds
  @return string