What Does “Day” Mean?

Todd and Craig had been exploring and mapping the wilderness for some time when they decided to split up to cover more ground. “You head to the north, and I will explore the south,” said Todd. “We’ll meet back at the campsite in two days.” Craig gave an affirmative nod and headed north. After two days of successful exploration, Todd returned to the campsite on schedule. But Craig was nowhere to be found. Perhaps he had been delayed.