When I tell the nanny I won't need her after 3pm even though she normally works until 5, or until 10 when she normally starts at 8am, then either:
1. If it's last minute I eat the cost and she gets paid anyway, or
2. If we have a little notice this is coming, I say, "We won't need you until noon next Thursday, would you like to work a date night next Friday night?"