Add a QueryManager for a specific timeframe.

def add_timeframed_query_manager(sender, **kwargs):
    """
    Add a QueryManager for a specific timeframe.

    """
    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))
    ))