At what time of day was Jesus crucified? Mark 15:25, 18 KJV


  And it was the third hour, and they crucified him.


and


  And began to salute him, Hail, King of the Jews!


John 19:14-16 KJV


  And it was the preparation of the passover, and about the sixth hour: and he saith unto the Jews, Behold your King!
  But they cried out, Away with him, away with him, crucify him. Pilate saith unto them, Shall I crucify your King? The chief priests answered, We have no king but Caesar.
  Then delivered he him therefore unto them to be crucified. And they took Jesus, and led him away.


How can Jesus be crucified on the 3rd hour but Pilate asked for his crucification at the 6th hour?
John operates on a different chronology than Mark.  

Matthew 26:17, Mark 14:12, Luke 22:7-9 all have the disciples asking Jesus where to prepare the Passover supper on first day of the feast of unleavened bread, with the crucifixion the next day.   But John 19:14 has the crucifixion itself taking place on "the preparation of the passover" which is the first day of the feast of unleavened bread.   So the day that Matthew, Mark, and Luke tell us Jesus had the disciples prepare the Passover, is the day he was crucified according to John.  So John makes the whole thing take place a day earlier than the Synoptics.  Once you realize that, it won't seem so important to be stuck on a difference of 3 hours.

There's an obvious theological reason why John does this. We are told in Mark 14:12 that the passover lamb is killed on the preparation day


  "And the first day of unleavened bread, when they killed the passover, his disciples said unto him, Where wilt thou that we go and prepare that thou mayest eat the passover?"


In the Synopics, then, Jesus' death does not correspond directly to the killing of the Passover lamb.  Rather, on the day the lamb is killed, the disciples are preparing for the Last Supper, and Jesus then eats an actual Passover seder with the disciples that night.

But John, who alone among the gospel writers speaks of Jesus as "the Lamb of God" (John 1:29, 1:36), wants Jesus' death to correspond exactly to the time that the Passover lamb is killed.   So John moves the Last Supper to a day earlier, thus having Jesus' crucifixion on the day of preparation when the lamb is killed.  Or does he?

The question is which is historical and which is merely theological spin.  Its possible the Synoptics are historical here and John is the spin because he wants Jesus' death to correspond exactly to the Passover lamb.  However, its also possible that John is historical and the Synoptics are spinning because they wanted Jesus to eat a real Passover meal as his last meal. [In John he's crucified while Passover is being prepared, so he doesn't get to eat a Passover meal.]  You will note that in John, there is no institution of the Lord's Supper (Communion, Eucharist) at the Last Supper as in the Synoptics.  Rather, John deals with this subject in John 6 when he has Jesus declare in the Synagogue at Capernaum that he is the bread from heaven.  It could be that the Synoptics moved the date of Jesus' crucifixion in order to accommodate having Jesus institute the Lord's Supper at the Last Supper during a real Passover, or it could be that John moved the date in order to have Jesus' death correspond to the killing of the pascal lamb, and we'll never know this side of heaven which it is.