The earliest source stating 25 December as the date of birth of Jesus is likely a book by Hippolytus of Rome, written in the early 3rd century.[45] He based his view on the assumption that the conception of Jesus took place at the Spring equinox which Hippolytus placed on 25 March, and then added nine months to calculate the date of birth. That date was then used for the Christmas celebration.[46] 25 March would also roughly be the date of his crucifixion, which ancient Christians would have seen as confirming the date of his birth, since many people of that era held the belief that the great prophets were conceived into the afterlife on the same date they were conceived into the world.[citation needed] John Chrysostom also argued for a 25 December date in the late 4th century, basing his argument on the assumption that the offering of incense mentioned in Luke 1:8–11 refers to the offering of incense by a high priest on Yom Kippur (early October), and, as above, counting fifteen months forward. However, this was very likely a retrospective justification of a choice already made rather than a genuine attempt to derive the correct birth date.[47] John Chrysostom also writes in his homily on the Nativity of our Lord Jesus Christ (Εἰς τὸ γενέθλιον τοῦ Σωτῆρος ἡµῶν Ἰησοῦ Χριστοῦ) that the date of 25 December was well known from the beginning among Westerners.[48]