code: def test_old_dates_drop():
      df = pd.DataFrame(
          {"date": ["2020-01-01", "2005-05-02", "1999-12-31", None]}
      )
      converter = DateTimeSearchKeyConverter("date")
      df = converter.convert(df)
      dataset = Dataset("test", df=df)  # type: ignore
      dataset.meaning_types = {"date": FileColumnMeaningType.DATE}
      dataset._Dataset__remove_old_dates()
      assert len(dataset) == 3
  
imports: import ipaddress
  import random
  from datetime import date, datetime
  import numpy
  import pandas
  import pytest
  from pandas.testing import assert_frame_equal
  from upgini import Dataset, FileColumnMeaningType
  from upgini.errors import ValidationError
  from upgini.metadata import ModelTaskType, SearchKey
  from upgini.resource_bundle import bundle
  from upgini.utils.datetime_utils import DateTimeSearchKeyConverter
  from upgini.utils.email_utils import EmailSearchKeyConverter
  from upgini.utils.features_validator import FeaturesValidator
  from upgini.utils.warning_counter import WarningCounter
label_desc: Use image node_id 21 for calling a global function with example usage: test_old_dates_drop() without return types