There is some controversy about how long Jesus was on the cross. The issue is when the crucifixion began. Mark says “It was the third hour when they crucified him.” (Mk 15:25) The Jewish counting of hours began at six am. So, the third hour would begin at nine am.
Or, was John correct saying the crucifixion began at the sixth hour, noon? When the crowd told Pilate that anyone who claims to be a king opposes Caesar, he brought Jesus out and had him sit on the “judges bench.” John states, “It was the day of Preparation for the Passover, and it was about the sixth hour.” (Jn 19:14) All the synoptic gospels (Matthew, Mark and Luke) agree that Jesus died at about the “ninth hour,” that is, 3:00 pm.
Part of the debate results from the more recent translation of the time in the gospels. The translators render the “third hour” as “nine o’clock in the morning” to make it simpler to the contemporary reader. But the Greek original here reads En de hora trite which translates literally, “It was then [the] hour third.” For accuracy it seems to be better to translate this more literally, “It was then the third hour,” and then provide a footnote stating that the third hour began at nine am. The same would apply to John’s reporting of the hour, “The hour was about the sixth.”
New Testament scholar Craig Blomberg points out that the Jewish people split daylight hours into four three-hour segments. They ordinarily marked the time of an event by rounding the time up or down to the three hour segment. The third hour would commence at nine am, the sixth would commence at noon, and the ninth hour, at three pm.
Thus, Blomberg concludes, “it becomes plausible to interpret Mark’s ‘third hour’ to mean any time between 9 a.m. and noon.” Mark just rounds down to the “third hour” whereas John rounds up to the sixth. John’s rounding up is supported by the fact that he says it was “about the sixth hour” (John 19:14).
Given that Mark and John are approximating the time of Jesus’ death, and they both approximate that time to be some time in the second quarter of the day, we can conclude there is no contradiction.1
Thus, about the sixth hour, around noon, would be a good estimate of when Jesus was placed on the cross, and three hours seems to be the amount of time Jesus endured on the cross.
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1: https://www.catholic.com/magazine/online-edition/the-timing-of-jesus-death