Revelation Pets does not support half day bookings but there is a workaround.
You can book a dog in for a day and adjust the time in the clock to reflect half a day.
Set up a Special Rate with your half day rate and book the dog in as normal. The system will assume the full day rate, but you can override this with the half rate in the Cost section of the booking. If your customer is paying by credit, then you can apply a 0.5 credit to the half day booking to reflect the half day price.
To set up a half day Special Rate, go to Account Settings > Services & Costs > Special Rates.