Here is the bottom line. No one knows for sure when the four gospels were written. All we can do is make educated guesses. What we should do is look at the evidence without any theological or other biases.
My best guess is that Mark was written in the 50s, Matthew and Luke were written in the 60s and John was written in the 80s. I have looked extensively at the evidence, but the honest truth is that the evidence is indirect. The gospel we can date with the greatest confidence is Luke, as it was written after Paul was in jail in Rome but before he died. This puts Luke and Acts in about AD 63-64. The great majority propose that Mark for sure and probably Matthew were written before Luke based on the content. Mark seems quite early, so I have come to the opinion that it was written in the 50s, but cannot rule out the late 40s. Surely all three were complete before AD 70 when Jerusalem was destroyed, as Luke and Matthew have prophecies of this event which would not even make sense if they were written after the events–otherwise these gospels would not have been accepted by the Church. John was probably written some time after the anti-Christian decision at the Council of Jamnia, around AD 85. So, to me the most likely date for the writing of John is in the late 80s. Revelation was written in the second half of the 90s, but John most likely in the 80s, but we cannot rule out the 70s.
Could John still remember accurately what Jesus did fifty-five years after the events? I say why not? I am 62 and I can remember quite accurately where I went to high school, the names of my friends, what classes I took in high school, the address where I lived, my phone number, the jobs I held, the vacations I took, and all kinds of very specific things from that time. Why would John not be able to remember the things that Jesus said and did? What reason would we have to doubt, for example, that he could remember the great events in the life of his Lord, Jesus Christ. John was serving as an elder in Ephesus at the time. He was presumably still mentally competent. I believe that a reasonable person would conclude that John’s memory was still intact at the age of 75. We can also assume that he had told these stories time and time again for the previous fifty plus years. Likely he had written many of them down before penning the gospel we now have. Does anyone have evidence that John could not accurately recall what happened? Clearly, the early church, who knew John personally, believed that his gospel was reliable. They were in a far better position than we are to judge if what he wrote was reliable because there were disciples still alive who were eye-witnesses to the events John recorded. The most reasonable conclusion is that John’s gospel is a more or less reliable account by a person who sincerely wanted to record what had happened in the ministry of his Lord Jesus.
Remember that you are interacting with Muslims who do not believe that Jesus was crucified. Is there even the slightest possibility that John could not remember how Jesus died, since he was there at the execution? Could Peter and Jesus’ mother Mary also have been wrong about how Jesus died? Really, it is silly to discuss the reliability of John’s gospel, written when he was about seventy-five or at most eighty years old, when Muslims want us to believe that he got the means of the death of Jesus wrong. The document whose reliability ought to be in question here is the Qur’an, not the Bible!!! The Qur’an VERY OBVIOUSLY is not inspired, as it denies one of the most well-documented events of ancient history–the crucifixion of Jesus in Jerusalem under Pontius PIlate. We should put the proverbial shoe on the other foot and ask the real question, which is can anyone take the Qur’an seriously, in light of the evidence!