code: def df_period_monthly_idx():
      """Create timeseries with Period index, monthly frequency."""
      return pd.DataFrame(
          data={"y": [1, 1, 1, 1, 1]},
          index=pd.period_range(
              start="2000-01-01", freq="M", periods=5
          ),
      )
  
imports: import pandas
  import pytest
  from pandas.testing import assert_frame_equal
  from sktime.transformations.series.time_since import TimeSince
  from sktime.utils._testing.hierarchical import _make_hierarchical
label_desc: Use image node_id 5 for calling a global function with example usage: df_period_monthly_idx() and returns: pd