Some of the reasons that point to a second century date for John's Gospel include:-
There is evidence that John drew inspiration from Luke's Gospel1.
Luke, in turn, is considered to have relied in part on the works of the Jewish historian, Josephus, thus placing Luke's Gospel no earlier than the late nineties of the first century CE.
- John's Gospel did not appear to have been widely known until later in the second century:
Polycarp, bishop of Smyrna in the second century, never mentioned the Gospel of John and did not appear to know of it, although Irenaeus claimed that Polycarp had met the disciple John and was even taught by him (or by the Gospel author).
Ignatius, bishop of Antioch in the second century, and Justin Martyr, the contemporary Christian philosopher in Rome, did not know of it either.
As far as is known, the Gospel was not referred to as having been written by 'John'until lrenaeus' Against Heresies, dated about 180 CE.
1The Cambridge Ancient History (Volume XI The Imperial Peace A.D. 70-192): "
The author draws material from Mark and Luke; doubtless also from independent tradition, neither the extent nor historical value of such tradition would seem great."