John writes his Gospel
Late in life, the apostle John wrote a Gospel so that readers might believe that Jesus is the Christ, the Son of God, and have life in his name. It records the “I am” sayings and Jesus' long teaching to his disciples on the night he was betrayed.
but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.
John 20:31 (World English Bible)
John's Gospel opens before creation: “In the beginning was the Word, and the Word was with God, and the Word was God” (John 1:1). It adds much not found in the other Gospels, including the early Judean ministry, the raising of Lazarus, and the four Passovers on which Ussher's chronology of Jesus' ministry is built.
Dating notes
Early church writers say John wrote at Ephesus late in the first century; most date it about AD 85–95, though some argue for a date before AD 70. The date is an estimate; Scripture doesn't say when the book was written.