code: def test_convert_date_columns(df):
      """Ensure datetime data converted to string format"""
      df["date"] = pd.to_datetime(df["date"])
      df = convert_date_columns(df, date_format="%Y-%m-%d")
      assert df["date"].values[0] == "2014-01-01"
  
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 43 for calling a global function with example usage: test_convert_date_columns(df) without return types