Great question, Graham!

The first-century Jewish mind didn't work that way.

Today, we think exclusively. We think three days and three nights must include a full 72 hours. We exclude anything less than 72 hours as being three days and three nights.

The first-century Jewish world thought inclusively.

They would consider one hour of day as the whole day.

You can see this throughout Scripture. David came to his men in Ziklag “on the third day,” but then it says that David hadn't eaten for “three days and three nights” (1 Samuel 30:1, 12-13).

If we used modern, exclusive reckoning, then David would have arrived on the fourth day, after spending 72 hours without eating.

But that's not how they thought. David arrived on the third day, being less than 72 hours. But because he didn't eat for part of the first day, the full second day, and part of the third day, he could be said to have fasted three days and three nights.

It was a well-known Jewish idiom.

Therefore, if Jesus was on the Cross for one hour on Friday before sundown, all of Saturday, and a few hours on Sunday morning, He would be considered to have been in the ground "three days and three nights," by the same idiom used in David's situation.