code: def test_modified_time(self):
      self.assertIsInstance(
          self.hffs.modified(self.hf_path + "/data/text_data.txt"),
          datetime.datetime,
      )
      self.assertIsInstance(
          self.hffs.modified(self.hf_path + "/data"), datetime.datetime
      )
      # should fail on a non-existing file
      with self.assertRaises(FileNotFoundError):
          self.hffs.modified(
              self.hf_path + "/data/not_existing_file.txt"
          )
  
imports: import datetime
  import unittest
  from typing import Optional
  from unittest.mock import patch
  import fsspec
  import pytest
  from huggingface_hub.hf_file_system import HfFileSystem
  from huggingface_hub.utils import RepositoryNotFoundError, RevisionNotFoundError
  from .testing_constants import ENDPOINT_STAGING, TOKEN
  from .testing_utils import repo_name
label_desc: Use image node_id 15 for calling the HfFileSystemTests obj's underlying member method code with example usage: obj.test_modified_time() without return types