1 On Pilate's judgment and condemnation before the tribunal, they took the Lord Jesus Christ, about the sixth hour, and led Him away. “And He, bearing His cross, went forth into the place that is called Calvary, but in Hebrew, Golgotha; where they crucified Him.” What else, then, is the meaning of the evangelist Mark saying, “And it was the third hour, and they crucified Him,” but this, that the Lord was crucified at the third hour by the tongues of the Jews, at the sixth hour by the hands of the soldiers?
That we may understand that the fifth hour was now completed, and there was some beginning made of the sixth, when Pilate took his seat before the tribunal, which is expressed by John as “about the sixth hour;” and when He was led forth, and nailed to the tree with the two robbers, and the events recorded were enacted beside His cross, the completion of the sixth hour was fully reached, being the hour from which, on to the ninth, the sun was obscured, and the darkness took place, we have it jointly attested on the authority of the three evangelists, Matthew, Mark, and Luke. But as the Jews attempted to transfer the crime of slaying Christ from themselves to the Romans, that is to say, to Pilate and his soldiers, therefore Mark suppresses the hour at which Christ was crucified by the soldiers, and which then began to enter upon the sixth, and remembers rather to give an express place to the third hour, at which they are understood to have cried out before Pilate, “Crucify, crucify him”, that it not only may be seen that the former crucified Jesus, namely, the soldiers who hung Him on the tree at the sixth hour, but the Jews also, who at the third hour cried out to have Him crucified.
Source: Tractates on the Gospel of John (New Advent)