How do we know when the gospel of John was written?

John

“There are two views concerning the date of John’s Gospel:

1) The traditional view places the writing of John around A.D. 85 or later. This view is supported by a statement from Clement of Alexandria that John wrote to supplement the other Gospel accounts. This would place his writings later in the first-century, considering the traditional view that the other Gospel writers wrote before A.D. 70. It is also argued that John’s theology appears more developed, giving suspicion for a later date.

2) Recently, interpreters have suggested an earlier date, somewhere around A.D. 50 but no later than A.D. 70. It is argued that this view does not contradict Clement’s statement. Furthermore, a more developed theology does not imply a later date. For example, the theology of Romans is very developed, nevertheless it is dated around A.D. 57. Lastly, attention is given to John 5:2 where John uses “is” rather than “was” concerning the pool near the Sheep Gate. This may suggest a time before 70 when Jerusalem was destroyed.”

Source

Could John have been written before the other gospels?

According to Bart Ehrman in his new book Jesus, Interrupted the Gospel According to John was the last gospel written, approximately 25 years after Mark.

There is a good deal of evidence that it was written after the others.

Some biblical scholars think that this gospel was written in stages. According to this view, the first stage was written quite early, maybe 50–60 AD, and may have been a miracles or signs book. Then later it may have been modified and expanded by a different author. This link provides more information about this theory.