code: def test_groupby_first_datetime64(self):
      df = DataFrame(
          [(1, 1351036800000000000), (2, 1351036800000000000)]
      )
      df[1] = df[1].astype("M8[ns]")
  
      assert issubclass(df[1].dtype.type, np.datetime64)
  
      result = df.groupby(level=0).first()
      got_dt = result[1].dtype
      assert issubclass(got_dt.type, np.datetime64)
  
      result = df[1].groupby(level=0).first()
      got_dt = result.dtype
      assert issubclass(got_dt.type, np.datetime64)
  
imports: from datetime import datetime, timedelta
  import numpy
  import pytest
  import pytz
  import pandas
  from pandas import DataFrame, DatetimeIndex, Index, MultiIndex, Series, Timestamp, date_range, offsets
  import pandas._testing
  from pandas.core.groupby.grouper import Grouper
  from pandas.core.groupby.ops import BinGrouper
label_desc: Use image node_id 14 for calling the TestGroupBy obj's underlying member method code with example usage: obj.test_groupby_first_datetime64() without return types