code: def at(self, time_str):
          
          assert self.unit in ('days', 'hours') or self.start_day
          hour, minute = time_str.split(':')
          minute = int(minute)
          if self.unit == 'days' or self.start_day:
              hour = int(hour)
              assert 0 <= hour <= 23
          elif self.unit == 'hours':
              hour = 0
          assert 0 <= minute <= 59
          self.at_time = datetime.time(hour, minute)
          return self
docs: Schedule the job every day at a specific time.
  
          Calling this is only valid for jobs scheduled to run
          every N day(s).
  
          :param time_str: A string in `XX:YY` format.
          :return: The invoked job instance