40 Matthew continues thus: “And they set up over His head His accusation written, 'This is Jesus the King of the Jews.'” Mark, on the other hand, before making any such statement, inserts these words: “And it was the third hour, and they crucified Him.” For he subjoins these terms immediately after he has told us about the parting of the garments. This, then, is a matter which we must consider with special care, lest any serious error emerge. For there are some who entertain the idea that the Lord was certainly crucified at the third hour; and that thereafter, from the sixth hour on to the ninth, the darkness covered the land.
According to this theory, we should have to understand three hours to have passed between the time when He was crucified and the time when the darkness occurred. And this view might certainly be held with all due warrant, were it not that John has stated that it was about the sixth hour when Pilate sat down on the judgment-seat, in a place that is called the Pavement, but in Hebrew, Gabbatha. For his version goes on in this manner: “And as it was the preparation of the passover, and about the sixth hour: and he says unto the Jews, Behold your King!
But they cried out, Away with him, away with him! crucify him! Pilate said unto them, Shall I crucify your king? The chief priests answered, We have no king but Cæsar. Then delivered he Him therefore unto them to be crucified.” If Jesus, therefore, was delivered up to the Jews to be crucified when it was about the sixth hour, and when Pilate was then sitting upon the judgment-seat, how could He have been crucified at the third hour, as some have been led to suppose, in consequence of a misinterpretation of the words of Mark?
Source: The Harmony of the Gospels (New Advent)