code: def test_period_range_to_timestamp_conversion():
      df = pd.DataFrame(
          {
              "date": pd.period_range(
                  start="2020-01-01", periods=3, freq="D"
              )
          }
      )
      converter = DateTimeSearchKeyConverter("date")
      df = converter.convert(df)
      assert df["date"].dtype == "Int64"
      assert df["date"].iloc[0] == 1577836800000
      assert df["date"].iloc[1] == 1577923200000
      assert df["date"].iloc[2] == 1578009600000
  
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 9 for calling a global function with example usage: test_period_range_to_timestamp_conversion() without return types