code: def _check_inv_operation_for_date(self, item, timestamp, data_dict, data_index_dict):
          
  
          if item in data_index_dict:
              for item in data_index_dict[item]:
  
                  # startdate < timestamp
                  if const.START in data_dict[item] and not const.END in data_dict[item]:
                      if data_dict[item][const.START] < timestamp:
                          return True
  
                  # enddate > timestamp
                  elif not const.START in data_dict[item] and const.END in data_dict[item]:
                      if data_dict[item][const.END] > timestamp:
                          return True
  
                  # startdate > timestamp > enddate
                  elif const.START in data_dict[item] and const.END in data_dict[item]:
                      if data_dict[item][const.START] < timestamp \
                              and data_dict[item][const.END] > timestamp:
                          return True
  
                  # no startdate or enddate available
                  elif not const.START in data_dict[item] and not const.END in data_dict[item]:
                      return True
  
          raise KeyError
docs: Checks if the callsign is marked as an invalid operation for a given timestamp.
          In case the operation is invalid, True is returned. Otherwise a KeyError is raised.