At what hour was Jesus crucified?

Mark 15:25 and John 19:14-16

1. The third hour (Mark 15:25) - "And it was the third hour when they crucified Him."
2. The sixth hour (John 19:14-15) - "Now it was the day of preparation for the Passover; it was about the sixth hour. And he *said to the Jews, "Behold, your King!" 15They therefore cried out, "Away with Him, away with Him, crucify Him!" Pilate *said to them, "Shall I crucify your King?" The chief priests answered, "We have no king but Caesar."

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.1

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1. 1. Archer, Gleason, Encyclopedia of Bible Difficulties, p. 364.