I remember there being plans to add a __date field lookup to make this easier, but as it stands the “standard” way of doing it is
today_min = datetime.datetime.combine(datetime.date.today(), datetime.time.min)
today_max = datetime.datetime.combine(datetime.date.today(), datetime.time.max)
Invoice.objects.get(user=user, date__range=(today_min, today_max))