How old the dinosaur bones are

Some tourists in the Chicago Museum of Natural History are marvelling 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 73 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 seventy three million years old when I started working here, and that was four and a half years ago.”