code: def df_datetime_15mins_idx():
      """Create timeseries with Datetime index, 15 minute frequency."""
      return pd.DataFrame(
          data={"y": [1, 1, 1, 1, 1]},
          index=pd.date_range(
              start="2000-01-01", freq="15T", 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 2 for calling a global function with example usage: df_datetime_15mins_idx() and returns: pd