Much like Pokemon Sun & Moon, Pokemon Ultra Sun & Ultra Moon actually take place at different times during the day compared to each other. In Pokemon Ultra Sun, the internal time clock is actually identical to the clock registered on your 3DS. So if it's daytime in the real world, it will be daytime in Pokemon Ultra Sun. In Pokemon Ultra Moon, however, the internal time clock has been inverted. This means that when its day in the real world where you are at, it should be, if your 3DS Clock is registered correctly, night within your game. Outside of certain Pokemon appearing during either the daytime or nighttime, not much more is affected by this mechanic.