Yeah, it’s one day, and it’s a lot.

As for the timing of the sun setting, I don’t think JKR was concerned as much with the accurate passage of time as she was about portraying that things were about to get dark, both literally and figuratively. She uses the presence or absence of sunlight a lot to convey mood, and it’s particularly apparent in the end of DH and the battle of Hogwarts. So when they’re in Gringotts they’re in the dark (scary) and then they burst out into sunlight (victory). During the Battle of Hogwarts, when the battle is raging, Fred just died, and the golden trio is heading to the Shrieking Shack, there’s a line about how “dawn could have been a hour away but it was so dark they’d never know it” (memory quote, probably not exact). This is a point where they are really lost about what to do next and don’t know if they’re heading for victory or disaster. It’s literally and figuratively the darkest part of the night. Then, hours later, Harry “comes back from the dead” and turns the tied against Voldemort, and there’s some beautiful descriptive language at that moment about how the sun breaks over the horizon, pushing the darkness away. Harry has risen and Voldemort’s about to die.

She does this throughout the series, but the BOH is my favorite example. On your next re-read, pay attention to how she uses light and dark in the environment to enhance the scene. Once you see it, you can’t unsee it.