For these are not drunken, as ye suppose, seeing it is but the third
hour of the day.
And he called unto him two centurions, saying, Make ready two hundred
soldiers to go to Caesarea, and horsemen threescore and ten, and
spearmen two hundred, at the third hour of the night;
It appears from these two passages that the third hour was reckoned differently by the two groups, the first by Peter, a Jew, and the second by a Roman. In both cases they anchored their time by the "day" or "night."
My research about the use of "hour" in the gospel of John is not really conclusive. It's possible that it was self-evident to his readers, so he did not see the need to add "day" or "night." So it "just makes sense" that the sixth hour was noon and Jesus may have been hot after walking in the desert heat, and stopped to rest.
Now from the sixth hour there was darkness over all the land unto the
Here, the darkness is given a beginning and an end. This passage only makes sense if the darkness started 6 hours after sunrise, noon, and lasted for the next 3 hours. Thus, the darkness is given a striking accompaniment to Jesus' crucifixion. It doesn't make sense to start the darkness at 6pm and note that it went to 9pm, does it? Darkness normally starts at the closing of the day. :) It seems to me that John was using the same time as Peter and Matthew: starting the count from sunrise.