code: def test_fillna_nat(self):
      series = Series([0, 1, 2, NaT._value], dtype="M8[ns]")
  
      filled = series.fillna(method="pad")
      filled2 = series.fillna(value=series.values[2])
  
      expected = series.copy()
      expected.iloc[3] = expected.iloc[2]
  
      tm.assert_series_equal(filled, expected)
      tm.assert_series_equal(filled2, expected)
  
      df = DataFrame({"A": series})
      filled = df.fillna(method="pad")
      filled2 = df.fillna(value=series.values[2])
      expected = DataFrame({"A": expected})
      tm.assert_frame_equal(filled, expected)
      tm.assert_frame_equal(filled2, expected)
  
      series = Series([NaT._value, 0, 1, 2], dtype="M8[ns]")
  
      filled = series.fillna(method="bfill")
      filled2 = series.fillna(value=series[1])
  
      expected = series.copy()
      expected[0] = expected[1]
  
      tm.assert_series_equal(filled, expected)
      tm.assert_series_equal(filled2, expected)
  
      df = DataFrame({"A": series})
      filled = df.fillna(method="bfill")
      filled2 = df.fillna(value=series[1])
      expected = DataFrame({"A": expected})
      tm.assert_frame_equal(filled, expected)
      tm.assert_frame_equal(filled2, expected)
  
imports: from datetime import datetime, timedelta, timezone
  import numpy
  import pytest
  import pytz
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, Period, Series, Timedelta, Timestamp, date_range, isna, timedelta_range
  import pandas._testing
  from pandas.core.arrays import period_array
label_desc: Use image node_id 1 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_fillna_nat() without return types