The fact that "three days" is used by Hebrew idiom for any part of three days and three nights is not disputed; because that was the common way of reckoning, just as it was when used of years.  Three or any number of years was used inclusively of any part of those years was used inclusively of any part of those years, as may be seen in the reckoning of the reigns of any of the kings of Israel or Judah.

But, when the number of "nights" is stated as well as the number of "days", then the expression ceases to be an idiom, and becomes a literal statement of fact.

Moreover, as the Hebrew day began at sunset the day was reckoned from one sunset to another, the "twelve hours in the day" (John 11:9) being reckoned from sunrise, and the twelve hours of the night from sunset.  An evening-morning was thus used for a whole day of twenty-four hours, as in the first chapter of Genesis.  Hence the expression "a night and a day" in 2Cor. 11:25 denotes a complete day (Gr. nuchthemeron).

When Esther says (Est. 4:16) "fast ye for me, and neither eat nor drink three days", she defines her meaning as being three complete days, because she adds (being a Jewess) "night or day".  And when it is written that the fast ended on "the third day" (5:1), "the third day" must have succeeded and included the third night.

In like manner the sacred record states that the young man (in 1Sam. 30:12) "had eaten no bread, nor drunk any water, three days and three nights".  Hence, when the young man explains the reason, he says, "because three days agone I fell sick".  He means therefore three complete days and nights, because, being an Egyptian (vv. 11, 13) he naturally reckoned his day as beginning at sunrise according to the Egyptian manner (see Encycl. Brit., 11th (Cambridge) ed., vol. xi. p. 77).  His "three days agone" refers to the beginning of his sickness and includes the whole period, giving the reason for his having gone without food during the whole period stated.

Hence, when it says that "Jonah was in the belly of the fish three days and three nights" (Jonah 1:17) it means exactly what it says, and that this can be the only meaning of the expression in Matt. 12:40; 16:4.  Luke 11:30, is shown in Ap. 156.

In the expression, "the heart of the earth" (Matt. 12:40), the meaning is the same as "the heart of the sea", "heart" being put by the Fig. Metonymy (of the Subject), Ap. 6 for "the midst", and is frequently so translated.  See Ps. 46:2.  Jer. 51:1.  Ezek. 27:4, 25, 26, 27; 28:2.  It is used of ships when sailing "in the heart of the seas", i.e. in or on the sea.  See Ezek. 27:25, 26; 28:8; also of people dwelling in the heart of the seas, i.e. on islands (Ezek. 28:2).  Jonah uses the Heb. beten ( = womb) in the same way (2.2).