def METHOD_NAME(self, thumbnails_dir):
    current_time = time.time()
    for root, _, filenames in os.walk(thumbnails_dir):
        for filename in filenames:
            path = os.path.join(root, filename)
            modification_time = os.path.getmtime(path)
            if current_time - modification_time > self._days_alive_secs:
                os.remove(path)