I personally believe John wrote his gospel between the years A.D. 65 and A.D. 70. I think John went to Ephesus to pastor the church in that city after Timothy left to assist Paul who was awaiting execution in Rome (2Ti 4:21; Heb 13:23-24). While in Ephesus John wrote this gospel, even though evidence suggests that the other three gospels (Matthew, Mark and Luke) had already been written. And certainly he would have known they existed and been shown them if possible, and may have even possessed a copy. But in spite of the existence of those gospels there was a growing confusion about Jesus that alarmed John. People were saying all sorts of wrong things about who He was. Some taught that He was just a man, so He should be thought of as no more than a great teacher or prophet. Others taught something that was opposite from this: They believed He was an angel (Heb 1:4-14). In other words He was a spirit-being of some sort who merely appeared to be human, but He was not really a human. And in addition to the debate about the nature of Jesus there has always been confusion over how God could have a son. So by the late A.D. 60’s there were growing groups of people who believed in a very different Jesus than the one the Apostles had taught. By that time most or all of the original twelve had died (Heb 2:3-4), most during Nero’s persecution (A.D. 64-88) and some in distant lands, so there was almost no one left with the authority to correct such distortions. I think that’s why John, who by then was probably in his late 50’s or early 60’s, gathered his own notes of Jesus’ sayings and refreshed his memory of those precious years when he walked beside Him. Then he sat down and wrote an account that emphasized the things Jesus said about Himself. The other gospels had been written earlier, in a different spiritual climate, so they had not focused on those sayings to the same degree. They focused on Jesus’ miracles and His wonderful teachings. Though frankly, everything a person needs to know about Jesus is in all the gospels. All present Him as both the divine Son of God and an incarnated man, yet somehow the statements in those gospels were being ignored by those determined to distort this truth. So John wrote his own and opened it with an absolutely clear declaration about the divine origin and human incarnation of Jesus (Jn 1:1-18). Then he went on to describe certain signs Jesus performed that fulfilled Old Testament prophesies about Him. But above all, John was careful to report exactly what Jesus said about Himself.