1 John, 2 John and 3 John

We do not know when these letters where written. Their content indicates a later date. With the latest date of John's death set at A.D. 98, it appears likely that these letters were written in the A.D. 90's.

Sources:
Encountering the New Testament, Walter A. Elwell / Robert W. Yarbrough, 1998, Baker Books, page 368