"Hey, thanks for asking. Here's the deal: we work on a daily rate, because most of our projects demand our full attention for at least the majority of a day. In order to be fair to all our customers and to keep things manageable for us, we bill in one-day increments."

I don't really see why I'd say more than that. If a client kept pushing after that, I'd start saying things like "I understand; let's figure out how we can get your work bundled up efficiently into a day" (or, in my case, more likely: I'd bill a day for the time-consuming stuff and just let the single hours go).