lamaeq,
re: "Correct."
So what examples are you using to say that it was common to forecast or say that a daytime or a night time would be involved with an event when no part of a daytime or no part of night time could occur?
You can find this trivially in Jewish sources online. "Day" in Hebrew can refer to the period of light during the day or the entire "night and day" cycle used by the Jews. The use of "day and night" was used to clearly denote a "night and day" (full 24 hours, although without clocks this is based on the changing time of the sunset during the seasons). So, Jesus was saying three days (part of the 24 hour cycle), which includes any part of the indicated 24 hour period. So, before sunset on Friday was day one, Saturday (the Sabbath) was day two, and Sunday (which started at sunset on Saturday by our count) was day three.
You are trying to demand, in a wooden literal fashion, that the Bible follow stopwatch quality modern scientific accuracy according to ideal perfect modern Western accuracy. Let us speculate: Say Jesus was crucified on Friday and hurried into a temporary grave before the start of the Sabbath (Saturday) at sunset. At what point does it become "three days and three nights"? Okay, the nights would be (by our count), Friday, Saturday and Sunday nights, but the days would be a tiny bit of Friday (how much? We don't know, certain lack of cell phones and watches back then.), all of Saturday, all of Sunday, but then since all of the Resurrection "action" happened in the early morning (it appears, remember lacking those cell phones and watches required for those TV courtroom style time ticks.), that would mean, using the logic you appear to, that the third "day" didn't happen because that was definitely several hours before the vague time of death on Friday.
I'll go back to Matthew where you got the quote from: 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: (Mat 12:39)
What sign are you seeking? If you are questioning this, if the time Jonah spent in the whale was more or less by even a second from 72 hours, does it make the Bible false? Even if those "days and nights" are adjusted for the variance of the sunrise and sunset for that time of year? Or does that give a bit of leeway?
If this is honestly bothering you and you are a Christian, then I will give some testimony to you; let it go. You will get your answer, but it is not one you need now. I've beat my head against the Bible (speaking metaphorically) seeking understanding many times in my life. I've always gotten the answer when I needed it, not when I wanted it.
The question always revolves around: Do I trust God? Does this tiny bit of the Bible demand an immediate solution or it will cost me my faith? Do I need to agonize over it so? I've learned the answers are Yes, No and Maybe For A Bit (until I figure out it is something to let go of for now).