Heidegger's Bible Handbook: Harmony of the Gospels: Dispute over the Length of Christ's Ministry
6. The space of time between His Baptism and ascension variously disputed among ancient and more recent men.
Moreover, concerning the space of time that passed from the Baptism of Christ to His ascension into Heaven, there are a variety of opinions. The common opinion among the most ancient was that the whole ministry of Christ was completed in the interval of one year or Passover, with the occasion taken from this, that Matthew, Mark, and Luke make mention of one Passover. Irenæus was the first to observe that John enumerates multiple Paschal feasts in the ministry of Christ. But he did not from John define the correct number of Passovers: indeed, declining to traditions, which in his age had a great show of plausibility, he relates that all the Elders in Asia testified, that John related that Christ was indeed baptized in His thirtieth year, but did not begin to teach before His fortieth year, at which time He attained to the age of a teacher; and at length, after He had taught for a number of years, He arrived at the age of Elders, which was about the fiftieth year. But that obviously ἄδηλος/uncertain tradition has no appearance of truth, and is openly inconsistent with the Gospel history. In our Age also, Theologians and Chronologists are divided in opinion, while some admit only three Passovers during the ministry of Christ: others gather four out of the Evangelists: yet others, five, of whose number is Joseph Scaliger, de Emendatione, book 6, page 552; and, following him, Wilhelmus Langius, de Annis Christi, book 2, chapter 10, out of the Evangelists, and Sethus Calvisius, Isagoge Chronologica, chapter 49, out of the Canons of the Callipic cycle, and also by the ordinary motion of the Moon, busy themselves to elicit and demonstrate five Passovers.
 Wilhelmus Langius (1624-1682) was a Danish mathematician, astronomer, and jurist.  Sethus Calvisius (1556-1615) was a German music theorist, astronomer, and chronologer.  Callippus (c. 370-c. 300 BC) was a Greek astronomer. He attempted to construct an accurate lunisolar calendar of seventy-six years, consisting of nine hundred and forty lunations.