John Lightfoot on Matthew 3:16

[And Jesus being baptized.] I. That Christ conversed upon earth two-and-thirty years and a half (as many years as David lived at Jerusalem; compare 2 Samuel 5:5), is proved hence:--1. That he was baptized when he had now completed his twenty-ninth year, and had newly begun his thirtieth. That the words of Luke imply, He began to be about thirty years old. Which words, although they are applied by some Christians to I know not what large latitude,--yet in the Jewish schools, and among that nation, they would not admit, certainly, of another sense than we produce. For there this axiom holds, The first day of the year is reckoned for that year. And, questionless, Luke speaks with the vulgar. For let it be supposed that the evangelist uttered these words in some Jewish school, "N. was baptized beginning to be about thirty years old": how could it be understood by them of the thirtieth complete (much less of the thirty-first, or thirty-second, as some wrest it)? when the words beginning to be about, do so harmoniously agree with the said axiom, as scarcely any thing can do more clearly. 2. That, from his baptism to his cross, he lived three years and a half. This is intimated by the angel Gabriel, Daniel 9:27; "In the half of a week" (that is, in three years and a half) "he shall make the sacrifice and oblation to cease"; and it is confirmed from the computation in the evangelists, but especially in John, who clearly mentioneth four Passovers (chap 2:13, 5:1, 6:4, and 13:1) after his forty days' fast, and not a little time spent in Galilee.

II. Therefore, we suppose Christ was baptized about the feast of Tabernacles, in the month Tisri, at which time we suppose him born; and that John was born about the feast of the Passover, and at that time began to baptize. For when Christ lived two-and-thirty years and a half, and died at the feast of the Passover, you must necessarily reduce his birth to the month Tisri, and about the time of the feast of Tabernacles: and when John the Baptist was elder than he by half a year, you must necessarily suppose him born about the feast of the Passover. But of these things we have said something already.