code: def add_timeframed_query_manager(sender, **kwargs):
      
      if not issubclass(sender, TimeFramedModel):
          return
      if _field_exists(sender, 'timeframed'):
          raise ImproperlyConfigured(
              "Model '%s' has a field named 'timeframed' "
              "which conflicts with the TimeFramedModel manager."
              % sender.__name__
          )
      sender.add_to_class('timeframed', QueryManager(
          (models.Q(start__lte=now) | models.Q(start__isnull=True)) &
          (models.Q(end__gte=now) | models.Q(end__isnull=True))
      ))
docs: Add a QueryManager for a specific timeframe.