I just listened to two Ahmed Deedat debates (contra McDowell and contra Douglas), both of which featured a very similar pair of arguments regarding the sign of Jonah. Each debate features both arguments, with largely the same flourishes - though there were some differences.
The first argument is presented with a great deal of showmanship and buildup, but it boils down to this: Jesus said he would be like Jonah, Jonah was alive in the belly of the whale, therefore Jesus could not be dead in the tomb. The flaw of the argument is fairly obvious: Jesus did not say that the similarity was that he would be alive for three days, but that he would be buried for three days.
But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: for as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here. The queen of the south shall rise up in the judgment with this generation, and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here.
As you can see from reading the argument itself, the point is not that Jesus would be alive, but that he would be buried. This argument is about as weak as they come.
The second argument is that Jesus was not in the tomb for three full days and nights, but only only two full nights (Friday and Saturday) and one full day (Saturday). This argument is slightly stronger. Yet it is still problematic.
This argument is premised on understanding Jesus to be using the expression "three days and three nights" to mean "three full days and three full nights." However, that is simply the same term taken from Jonah 1:17, and there is no indication there that the term means precisely 72 hours. Indeed, there is no particular indication from the context of Jonah 1:17 as to what time of day Jonah was cast into the sea. We might surmise it was evening because he had gone to sleep, but the text does not tell us.
What else could the term mean? Well, it could mean "three consecutive days." The places where we find this idiom is in the context of the rain of the flood (40 consecutive days Genesis 7:4 and 12), Moses' fast during the time of the reception of the law and intercession for the people (Exodus 24:18 and 34:28; Deuteronomy 9:9, 11, 18, and 10:10), the fast of the captured slave (1 Samuel 30:12); Elijah's fast on the way to Horeb (1 Kings 19:8); Job's seven days of silence with his friends (Job 2:13); and Jesus forty day fast (Matthew 4:2). In each of these cases, the point of the idiom is the fact of an unbroken succession of days.
Thus, the forty day fast of Jesus (and Moses and Elijah) was not the like fast of the Muslims, who break their fasts in the evening. Instead, it was unbroken. The rain that flooded the whole world was not a month and a third of Seattle-like weather, it was 40 days of constant rain.
We even see a similar usage in the singular:
Esther 4:16 Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day: I also and my maidens will fast likewise; and so will I go in unto the king, which is not according to the law: and if I perish, I perish.
Psalm 1:2 But his delight is in the law of the LORD; and in his law doth he meditate day and night.
Isaiah 34:10 It shall not be quenched night nor day; the smoke thereof
shall go up for ever: from generation to generation it shall lie waste;
none shall pass through it for ever and ever.
Revelation 14:11 And the smoke of their torment ascendeth up for
ever and ever: and they have no rest day nor night, who worship the
beast and his image, and whosoever receiveth the mark of his name.
Many other passages have the same usage. See Leviticus 8:35, Deuteronomy 28:66, Joshua 1:8, 1 Kings 8:29 and 59, 1 Chronicles 9:33, 2 Chronicles 6:20, Nehemiah 1:6 and 4:9, Psalm 32:4, 42:3, 55:10, and 88:1, Ecclesiastes 8:16, Isaiah 27:3, 60:11, and 62:6, Jeremiah 9:1, 14:17, and 16:13, Lamentations 2:18, Mark 4:27 and 5:5, Luke 2:37 and 18:7, Acts 9:24, 20:31, and 26:7, 1 Thessalonians 2:9 and 3:10, 2 Thessalonians 3:8, 1 Timothy 5:5, 2 Timothy 1:3, Revelation 4:8, 7:15, 12:10, and 20:10.
The point is not 24 hour periods, but rather unbroken continuity. Deedat has misinterpreted "three days and three nights" to mean 72 hours, when rather it means three successive days (Friday, Saturday, and Sunday).
There is more to the rebuttal to Deedat, though. Part of the sign of Jonas is that Jesus is greater than Jonah. In fact, in the Luke account, the days in the tomb are not even mentioned. How was Jonah a sign to the Ninevites? He was like a man who had come back from the dead, having been spit out by the great fish that swallowed him.
Christ however, is much greater than Jonah, in that he really did come back from the dead. Likewise, while Solomon was the wisest man, Jesus is greater than Solomon for Jesus is God.
And when the people were gathered thick together, he began to say, This is an evil generation: they seek a sign; and there shall no sign be given it, but the sign of Jonas the prophet. For as Jonas was a sign unto the Ninevites, so shall also the Son of man be to this generation. The queen of the south shall rise up in the judgment with the men of this generation, and condemn them: for she came from the utmost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here. The men of Nineve shall rise up in the judgment with this generation, and shall condemn it: for they repented at the preaching of Jonas; and, behold, a greater than Jonas is here.
If only Deedat had read the Scriptures with eyes opened by the Holy Spirit. But he did not. Dear readers, do not follow his bad example of misunderstanding of the Sacred text. Instead, properly understand the Scriptures and learn from them about the sign given to that adulterous generation and handed down to our adulterous day.
P.S. It was interesting to see that Dr. Douglas used some arguments around 1 hour, 42 minutes into the debate regarding the use of skeptics by Muslims - it reminded me of the arguments my friend Dr. White (who pointed me to the McDowell debate) uses.