class_docstr: Date Operator
  
  Parameters
  ----------
  feature : Expression
      feature instance
  
  Returns
  ----------
  feature:
      a series of that each value is the date corresponding to feature.index
class_docstr_tok: ["Date","Operator","Parameters","--","--","--","--","--","feature",":","Expression","feature","instance","Returns","--","--","--","--","--","feature",":","a","series","of","that","each","value","is","the","date","corresponding","to","feature.index"]
code: def _load_internal(self, instrument, start_index, end_index, freq):
      _calendar = get_calendar_day(freq=freq)
      series = self.feature.load(
          instrument, start_index, end_index, freq
      )
      return pd.Series(_calendar[series.index], index=series.index)
  
imports: import numpy
  import pandas
  import importlib
  from qlib.data.ops import ElemOperator, PairOperator
  from qlib.config import C
  from qlib.data.cache import H
  from qlib.data.data import Cal
  from qlib.contrib.ops.high_freq import get_calendar_day
label_desc: Use image node_id 1 for calling the Date obj's underlying member method code with example usage: obj._load_internal(instrument, start_index, end_index, freq) and returns: pd