docs: def to_timestamp(self, data):
          """Transform a datetime series into linux epoch.
  
          Args:
              data(pandas.DataFrame): DataFrame containins a column named as `self.col_name`.
  
          Returns:
              pandas.Series
          """
          result = pd.Series(index=data.index)
          _slice = ~data[self.col_name].isnull()
  
          result[_slice] = data[_slice][self.col_name].astype('int64')
          return result