Barrett suggests an earliest date of 90, based on familiarity with Mark’s gospel, and the late date of a synagogue expulsion of Christians (which is a theme in John).
There are critical scholars who are of the opinion that John was composed in stages (probably two or three), beginning at an unknown time (50-70? This date is assumed in large part because John 21, the so-called "appendix" to John, is largely concerned with explaining the death of the "beloved disciple," supposedly the leader of the Johannine community that would have produced the text.
If this leader had been a follower of Jesus, or, more likely, a disciple of one of Jesus' followers, then a death around 90-100 is possible.
The earliest known manuscripts of the New Testament is a fragment from John, P52. The difficulty of fixing the date of a fragment based solely on paleographic evidence allows for a range of dates that extends from before 100 to well into the second half of the second century.
This hypothetical "Signs Gospel" is alleged to have been circulating before 70.