[OrthodoxPresbyterian]

The difference in the way time was counted between the Jews and the Romans is not obvious from the Scriptures, is it?

Well, there's hints within the Gospel of John itself that John is using a different time scale; here's one:

We know from the Synoptics that the crucifixion took over 6 hours. If John's sixth hour is really the Jewish sixth hour - noon, as unfortunately, even the Living Bible says - then the crucifixion lasted past the time when the Sabbath started. John 19:31 says that the Jews didn't want the bodies left up over the Sabbath, which obviously means that the Sabbath hadn't started yet. So either John is giving us an extraordinarily short crucifixion, or he is giving us the time in Roman. Since crucifixions were usually extended affairs, the latter assumption is more valid.
http://www.tektonics.org/passovertime.html

But, admittedly, you have to look for such hints; so I should give you credit for the fact that John's time-scale is different is not immediately apparent.

However, there is a very obvious indication that John was using a different time-scale from the Synoptics -- the fact that the Early Church canonized both John and the Synoptics despite the (apparent) difference in hour-counts. We moderns have a tendency to think ourselves smarter than our forebears, but that just ain't the case -- if there were an obvious discrepancy between John and the Synoptics which did not arise from some identifiable difference in measurement (that is, John's use of Roman time), do you really think that the Early Fathers would have canonized John along with the Synoptics? They threw out a lot of other early "pseudo-gospels" which conflicted with the Synoptics in one way or the other; if John's Gospel did conflict with the Synoptics, what makes you think they would have included it in the Canon?

That's one consideration we should always remember when approaching alleged Bible "discrepancies" -- our forebears were not, in fact, more dullardly than are we. If there's an apparent discrepancy between one book of the Bible and another, it behooves us to ask: Why did the ancients not see a discrepancy here? What did they see, that I'm not seeing?

Until we ask that question in regard to any apparent "contradictions" in Scripture, we haven't really done our homework.

[kosta50]

They threw out a lot of other early "pseudo-gospels" which conflicted with the Synoptics in one way or the other; if John's Gospel did conflict with the Synoptics, what makes you think they would have included it in the Canon?

Mainly because it's not a theological discrepancy. "Pseudo-gospels" were rejected on theological errors -- such as Gnostic narcissism or denying divinity of Christ or his dual natures.

The explanation of two different systems of time-keeping are not only likely but almost certain based on the historical knowledge of the times, but the reason I asked was to see if this could be explained on biblical and not profane knowledge.