When you house sit, each "night" is good for up to 24 hours of care. So, when you leave depends on when you arrived. Remember, you don't have to sit in the house all day. You are free to take care of other pets (walks or drop-ins) while house sitting. Just be sure the client understands what the service entails. And you set the rules. Personally, I feel your house sitting rate is rock bottom for a place like Boulder and the customer shouldn't expect 24 hour care for that amount.
Make sure to set the times (arrival and departure) before the job is booked. Then you can adjust the bill to reflect the actual amount of time you have to stay. Let's say, the customer wanted you to start the job at 7 pm, then you should be prepared to leave at or about 7 pm on the last day.
If you started in the morning and the customer wasn't coming home until the evening, then you could charge them for a couple of drop-ins as compensation for the extra "day."
Hope this puts it in perspective and gives you a better idea of how you should be in control of your bookings.
Its a 24 hour period (12pm the 13th to 12pm the day of return.) Any time over that amount is solely up to you on your leeway of time or if over a certain amount of time then a daycare fee could be added.
You can't add a daycare charge when the person is house sitting.