It’s not a dumb question! I count hours, because not everyone fasts at the same time so the sunrise/sunset thing is kind of hard to consider. Hours are also useful given there’s marks you hit in dryfasting often at certain times and it can help you monitor that
I actually count after a full nights sleep because that’s where the magic happens, for instance if I stop feeding at 9 pm today, the fast begins for me the next morning so the next day at 9 pm Is not one full day….it becomes one full day for me after sleeping through the night till the next morning because the night sleep is where the most healing and deep breathing happens so I rather count this the way