'Days' in the Bible

Yahweh prepared a great fish to swallow up Jonah, and Jonah was in the belly of the fish three days and three nights.

So Jonah arose, and went to Nineveh, according to the word of Yahweh. Now Nineveh was an exceedingly great city, three days' journey across.

Jonah began to enter into the city a day's journey, and he cried out, and said, "In forty days, Nineveh will be overthrown!"