code: def test_convert_date_columns_default(df):
      """Tests default datetime format for dataframe date serialization
      remains as numpy datetime format"""
      df["date"] = pd.to_datetime(df["date"])
      df = convert_date_columns(df)
      assert isinstance(df["date"].values[0], numpy.datetime64)
  
imports: import os
  import json
  import numpy
  import pytest
  import pandas
  from pandas.util.testing import assert_frame_equal
  from matplotlib.pyplot import imread
  from mapboxgl.errors import SourceDataError, DateConversionError
  from mapboxgl.utils import df_to_geojson, geojson_to_dict_list, scale_between, create_radius_stops, create_weight_stops, create_numeric_stops, create_color_stops, img_encode, rgb_tuple_from_str, color_map, height_map, numeric_map, convert_date_columns
label_desc: Use image node_id 44 for calling a global function with example usage: test_convert_date_columns_default(df) without return types