In those days John the Baptist came, preaching in the wilderness of Judea
Matthew 3:1 Now in those days Luke 3:1. And in the fifteenth year It could not be gathered from Matthew and Mark in what year of his age John began to preach: but Luke shows sufficiently, that he was about thirty years of age. The ancient writers of the Church are almost unanimously agreed, that he was born fifteen years before the death of Augustus.
We have here an account of the preaching and baptism of John, which were the dawning of the gospel-day. Observe, I. The time when he appeared. In those days (Mat 3:1), or, after those days, long after what was recorded in the foregoing chapter, which left the child Jesus in his infancy.
Commenting on Matthew 3:1-6
MATTHEW CHAPTER 3 Verse 1. In those days. The days here referred to cannot be those mentioned in the preceding chapter, for John was but six months older than Christ. Perhaps Matthew intended to extend his narrative to the whole time that Jesus dwelt at Nazareth; and the meaning is, "in those days while Jesus still dwelt at Nazareth." John began to preach.