code: def str_to_ts(dt_str):
      return pd.Timestamp(dt_str, tz="UTC")
  
imports: import pandas
  from pandas.util.testing import assert_series_equal
  from six import iteritems
  from functools import partial
  from toolz import groupby
  from zipline.finance.asset_restrictions import RESTRICTION_STATES, Restriction, HistoricalRestrictions, StaticRestrictions, SecurityListRestrictions, NoRestrictions, _UnionRestrictions
  from zipline.testing import parameter_space
  from zipline.testing.fixtures import WithDataPortal, ZiplineTestCase
label_desc: Use image node_id 1 for calling a global function with example usage: str_to_ts(dt_str) and returns: pd