AniGa, depends on your definition of a day. If a day to you mean 24 hours, then use the solar day. A solar day is determined (correct me if I'm wrong) by the time it takes earth to rotate once in reference to the sun. This is slightly longer or shorter (based on which way the planet is rotating) than a rotational period, which is a rotation in relation to the stars. It's different because when the earth rotates once, it would have also traveled along its orbit too, so the angle of a line from the earth to the sun would change a little to. This means that the time of each solar noon would happen at a different rate than noon for a star outside the solar system. Obviously this difference would add up, and they do. That's why constellations appear at slightly different places each day. I hope I explained this well enough.
If you mean to find the length of the day as in sunrise to sunset, that is not constant across a planet. For example, a "day" in Antarctica would be 6 months.
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