Accuracy of Rise/Set Computations
Actual times of sunrise and sunset depend on unpredictable atmospheric conditions that affect the amount of refraction at the horizon. Thus, even under ideal conditions, the times computed for sunrise and sunset may be in error by a minute or more.
The accuracy of sunrise and sunset computations decreases at high latitudes. There, small variations in atmospheric refraction can change the times of rising and setting by many minutes, since the Sun intersects the horizon at a very shallow angle.