John wrote his gospel after the others, and it is definitely different (the other three, being very similar, are called the Synoptic Gospels). He didn't leave things out because he didn't believe them, he left them out because they weren't necessary to write. He gave his purpose for writing this latter gospel (20:31), and admitted he didn't include everything (20:30). The other gospels already covered the crucifixion and other things; John's was written to actually cover things that had been left out of the others. In the Archaeological Study Bible I have, it says that John's audience were the non-Jewish believers who struggled with the popular Greek philosophies, that is Gnosticism, that claimed Jesus was divine, but not human. John's letters also deal with this issue.
This to me is problematic, as it just sounds too convenient, that another piece of the jigsaw should arrive to further the Churches claim to make Jesus Godlike. Some of the events that 'John' covered are not in the other three Gospels, why is this? The Gospel of John was written in about 80AD, this would have made John about 75 years of age give or take at the time of writing. If you areached the age 60 you were doing well in this era, 70 was not impossible, but to write a memoir on something 50 years would be extreme.
John's Gospel is usually dated very late--toward the end of the century--but there are reasons for believing that it was actually written much earlier. The John Rylands Papyrus* suggests that John was already in wide circulation during the second century A.D. . . . Some have even proposed a date prior to the destruction of the temple in A.D. 70 . . . . It has been suggested that John may have written from Ephesus.
But this piece of papyrus has also been disputed Vicki, Andreas Schmidt, for example, in comparing different forms of handwriting actually places the date of the papyrus at around 170 AD. Most other non-biblical scholars would place it in 90AD and most New Testament Scholars would disagree, but this again goes back to my point on being biased and unbiased on each side. You might believe (and you might be indeed right), that you are analyzing something the correct way, but the prcocess could be flawed. That goes for all sides.
This papyrus "is the oldest copy yet discovered of any portion of the New Testament, dating back to the first half of the second century A.D." (ASB p. 1755). It was from Egypt, so the Gospel of John was already there and being read at this early date, and this after it was copied and transferred to Egypt, which could have taken quite a few years, even up to 30.
The dates you have given me here still places the Gospel of John circa 80AD, or even later.
Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying. Arthur C. Clarke