Dinosaur bones

Some tourists in the Chicago Museum of Natural History are marveling at the dinosaur bones. One of them asks the guard, "Can you tell me how old the dinosaur bones are?” The guard replies, "They are 3 million, four years, and six months old."
"That's an awfully exact number," says the tourist. "How do you know their age so precisely?"

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