code: def test_datetime64_fillna_backfill(self):
      # GH#6587
      # make sure that we are treating as integer when filling
      ser = Series(
          [NaT, NaT, "2013-08-05 15:30:00.000001"], dtype="M8[ns]"
      )
  
      expected = Series(
          [
              "2013-08-05 15:30:00.000001",
              "2013-08-05 15:30:00.000001",
              "2013-08-05 15:30:00.000001",
          ],
          dtype="M8[ns]",
      )
      result = ser.fillna(method="backfill")
      tm.assert_series_equal(result, 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 15 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_datetime64_fillna_backfill() without return types