Dinosaur dating

Some tourists in the Chicago Museum of Natural History were marveling at the dinosaur bones. One of them asked the guard, "Can you tell me how old the dinosaur bones are?"

The guard replied, "They are 3 million, four years, and six months old."

"That's an awfully exact number," said the tourist. "How do you know their age so precisely?"

The guard answered, "Well, the dinosaur bones were three million years old when I started working here, and that was four and a half years ago."