Most probably, John was using the Roman measurement of time when dealing with the crucifixion. Matthew, Mark, and Luke, for the most part, used the Hebrew system of measuring a day: from sundown to sunup. The Roman system was from midnight to midnight. "John wrote his gospel in Ephesus, the capital of the Roman province of Asia, and therefore in regard to the civil day he would be likely to employ the Roman reckoning. (Encyclopedia of Bible Difficulties, by Gleason Archer, page 364.)