Vico,
re: “The norm is that a day is sunset to sunset, beginning with night then day…”
That is correct.
re: “…so three can mean part of day 1, even just before sunset, through part of day 3, such as just after sunrise, giving a period of slightly more than 36 hours.”
Correct again. What is your point?
Matthew 12:38 Then some of the Scribes and Pharisees answered him, saying: Master, we would see a sign from thee.
39 But he answering, said to them: *An evil and adulterous generation seeketh for a sign: and a sign shall not be given it, **but the sign of Jonas, the prophet.
40 For as Jonas was in the whale’s belly three days and three nights: so shall the Son of man be in the heart of the earth three days and three nights.
Haydock Commentary:
Ver. 40. In the whale’s belly.[4] The word signifies a great fish, and was not perhaps that which we commonly call a whale. In the prophet Jonas, it is called, a great fish.
— Three days and three nights; not three whole days and three nights, but part of three natural days, from which, in common computation, the nights used not to be separated. We have an instance of this, Esther iv. 16, where the Jews were ordered to fast with her three days, and three nights: and yet (Chap. v, ver. 1) Esther, after part of three days, went to the king.
— In the heart of the earth: by which is signified, Christ’s descent into hell; as St. Paul says (Ephesians iv. 9.) that he descended into the inferior parts of the earth, and this cannot be understood of the grave only. (Witham)
— Jesus Christ expired on the cross about the ninth hour, or 3 p.m. when the general and supernatural darkness that covered the earth, may be counted for the first night, and the light which again appeared, for the term of the first day. (Bible de Vence)
— As Jonas was a sign to the Ninivites, so is Christ to the Jews; for as he by the prodigy of remaining so long in the fish’s belly, and afterwards coming forth alive, gave such authority to his preaching, that the Ninivites were converted; so Christ, by his death and resurrection on the third day, shall shew that he is the true Christ, and this generation shall acknowledge him for the Messias. (Menochius)