This one takes more digging in the Bible. This difference, involves the Jewish times of the day, and our times of the day (the Roman time of day: 12am-11:59pm). Let's look at the Jewish times of day in the New Testament time: (hours are approximate because the length of the hour was not "set in stone" until about the 18th century)
In Mark 15:25 we see Jesus crucified in the Third Hour, which is about from 6am-9am. In John 19:14 it appears to be the Sixth hour, 9am-12pm, when Pilate brought Jesus before the crowd, right before Jesus was sentenced to Crucifixion! Is there a contradiction? It honestly appears to be . . . but it's not! The difference is the system of time that Mark wrote in, and John wrote in! Mark, wrote in the Jewish times of day, while John wrote in the Roman times of day! That might seem like a long-shot, but let's look at the context (not to mention, more highly studied Biblical scholars have said Mark seemed to write more after the Jewish manner, and John more after the Gentile/Roman manner) to check if this makes sense.
Look at the time frame for this; Peter denied Jesus, The cock crew, then we see Jesus being led to the chief priests STRAIGHTWAY in the Morning (as said in Mark), and EARLY in the Morning (as said in John). Now for both Jewish times and Roman times, Morning and Night are the same, it is still dark at night and light during the day. The only difference is when they consider the daily time frame to begin. Morning, is Midnight and onward, from the time the moon begins to descend and give way to the sun. Something to note, is that the Jewish timing, counts also the Third Watch (12am-3am), as "cockcrow," and the Fourth Watch (3am-6am) as "morning." Though roosters crow throughout the day (believe me, I know, I used to live by one!) they are most noted for crowing in the early hours of the morning, this probably caused the Jews to also think of the Third Watch as "cockcrow," and honestly is probably the time when Peter heard it after denying Jesus! Then it says they led Jesus in the early morning, or straightway in the morning, which would then be considered perhaps as the Fourth Watch (also known as "morning" the way Third Watch is known as "cockcrow"), between 3am-6am. Then we see Jesus Crucified (in Mark) DURING the Third Hour, from 6am-9am, and in John at the "Sixth Hour" about to be led to the hill to be crucified, which knowing to be the Roman time of day, is about the beginning of the "Third Hour" at 6am.
All that, if we understand how John wrote in Roman time of day, and Mark in Jewish time of day, many other things make sense, even from the inscription on the cross, Mark says: "The King of the Jews," and John says, "Jesus of Nazareth The King of the Jews" It was written in three languages: Greek, Latin, and Hebrew. I believe Mark wrote the Hebrew form, John the Greek form. There is a clear difference between John's form, and Mark's form, because they probably did write, Mark, Hebrew form, and John, the Greek (or maybe Latin) form. Another thing that makes sense in light of all this, is that in John it is mentioned the "Seventh hour" (John 4:52). Unless it's mistaken, there was no "seventh hour" in New Testament Jewish time of day, but indeed there is in Roman time of day!
Another thing that makes sense when we see that John wrote in Roman time, and Mark wrote in the Jewish time, is the fact that the Jews were rushing so they could eat the Passover meal and finish before morning or daylight (which Jesus being condemned in the area of 6am, and crucified in the Third Hour (6am-9am) that would have been possible). They are commanded to do so early and be ready to leave by morning . . . remembering the commandments from Exodus. Now, if John were speaking in terms of Jewish times of day, Jesus would be before Pilate, about to be condemned, from 9am-12pm. That would be far too late in the day. It would actually be close to the END of that day, and close to the BEGINNING of the Sabbath day, wherein everyone was to rest, not still deal with the passover work.
These are a few clues in the Bible, that show how Mark and John still agree, yet Mark speaks in terms of the Jewish times of day, and John speaks in terms of the Roman times of day. The truth of the matter: Jesus was about to be condemned at about 6am or so, and He was Crucified a little later on, between 6am and 9am (probably closer to the beginning half of the "Third Hour").