code: def test_calendar_time_trend_base(time_index):
      ct = CalendarTimeTrend(
          MONTH_END, True, order=3, base_period="1960-1-1"
      )
      ct2 = CalendarTimeTrend(MONTH_END, True, order=3)
      assert ct != ct2
      str(ct)
      str(ct2)
      assert ct.base_period is not None
      assert ct2.base_period is None
  
imports: from statsmodels.compat.pandas import MONTH_END, PD_LT_1_0_0, QUARTER_END, YEAR_END, is_int_index
  from statsmodels.compat.pytest import pytest_warns
  from collections.abc import Hashable
  import numpy
  import pandas
  import pytest
  from statsmodels.tsa.deterministic import CalendarFourier, CalendarSeasonality, CalendarTimeTrend, DeterministicProcess, DeterministicTerm, Fourier, Seasonality, TimeTrend
label_desc: Use image node_id 11 for calling a global function with example usage: test_calendar_time_trend_base(time_index) without return types