41 First, then, let us consider what the hour really is at which He can have been crucified; and then we shall see how it happens that Mark has reported Him to have been crucified at the third hour. Now it was about the sixth hour when Pilate, who was sitting, as has been stated, at the time upon the judgment-seat, delivered Him up to be crucified. The expression is not that it was the sixth hour fully, but only that it was about the sixth hour; that is to say, the fifth hour was entirely gone, and so much of the sixth hour had also been entered upon.
These writers, however, could not naturally use such phraseologies as the fifth hour and a quarter, or the fifth hour and a third, or the fifth hour and a half or anything of that kind. For the Scriptures have the well-known habit of dealing simply with the round numbers, without mention of fractions, especially in matters of time. We have an example of this in the case of the “eight days,” after which, as they tell us, He went up into a mountain, — a space which is given by Matthew and Mark as “six days after,” because they look simply at the days between the one from which the reckoning commences and the one with which it closes.
This is particularly to be kept in view when we notice how measured the terms are which John employs here. For he says not “the sixth hour,” but “about the sixth hour.” And yet, even had he not expressed himself in that way, but had stated merely that it was the sixth hour, it would still be competent for us to interpret the phrase in accordance with the method of speech with which we are, as I said, familiar in Scripture, namely, the use of the round numbers. And thus we could still take the sense quite fairly to be that, on the completion of the fifth hour and the commencement of the sixth, those matters were going on which are recorded in connection with the Lord's crucifixion, until, on the close of the sixth hour, and when He was hanging on the cross, the darkness occurred which is attested by three of the evangelists, namely, Matthew, Mark, and Luke.
Source: The Harmony of the Gospels (New Advent)