As soon we we start talking timing, I let clients know that although Rover's calculations do their automatically calculate per night, we charge $X per day, since that's where we're putting in our time and energy with their dog. I've never had someone object, and many understand and assume that makes sense. If you're having trouble with that expectation in your area, I'd recommend writing it in your profile so that owners who read it get a feel for that before the conversation.