The hours of the cross
Whether Jesus was crucified at the third hour, as Mark says, or was still before Pilate at the sixth, as John.
Mark 15:25
25It was the third hour, and they crucified him.
John 19:14
14Now it was the Preparation Day of the Passover, at about the sixth hour. He said to the Jews, “Behold, your King!”
What each tradition says
Rabbinic / Midrashic
No sourced position found. Checked the Jewish Encyclopedia (1905), articles "Jesus of Nazareth" (Passion narrative — no discussion of the third vs sixth hour), "New Testament" and "Simon Cephas"; Sefaria holds no Gospel commentary. No rabbinic position on the hour found.
Patristic
Augustine says such impious pertinacity should perish. He says it is to be believed that the Lord Jesus Christ was crucified at once at the third hour by the voice of the Jews and at the sixth by the hands of the soldiers.
Augustine, Harmony of the Gospels (Nicene and Post-Nicene Fathers, First Series, Vol. 6; S.D.F. Salmond trans.) — Book III, ch. 13 (§47)
Catholic
Aquinas says the third hour was when the Jews clamored for the Lord to be crucified. In clamoring, they are most clearly shown to have crucified Him. Thus "It was the third hour, and they crucified Him" keeps the crime on the Jews rather than the soldiers, because the Jews first clamored for His crucifixion at that hour.
Thomas Aquinas, Summa Theologiae (Fathers of the English Dominican Province trans., second and revised edition) — III, q. 46, a. 9, ad 2
Orthodox
No sourced position found. Checked archive.org for a public-domain English Theophylact or Euthymius Zigabenus commentary on John (none); Philaret’s Longer Catechism; Acts of the Synod of Jerusalem (no passage commentary); pravoslavieto.com (no English commentary on John 19). No Eastern treatment of the third/sixth hour found.
Reformed / Protestant
Calvin says the day was divided into four parts, as the night also had four watches. In consequence, the Evangelists sometimes allotted only four hours to each day, extended each hour to three, and reckoned an hour drawing to a close as belonging to the next part.
John Calvin, Commentary on the Gospel according to John, Vol. 2 (William Pringle trans., 1860) — On John 19:14
Evangelical
According to Barnes, the Jews divided night and day into four equal three-hour parts. The first daytime division ran from six to nine in the morning; the second from nine to twelve. So Mark’s third hour corresponds to nine o’clock, and John’s sixth hour to twelve, or noon.
Albert Barnes, Albert Barnes’ Bible Commentary — On Mark 15:25