The "Precious" coin

Once a man found a old- looking coin. On one side it had a picture of a queen, and where the year of manufacture was usually written, it was written 20 BC. Yipee! the man cried. If it was that old, it was worth a lot of money. He took it to a museum to sell it. When the museum clerk took one look at the coin, he laughed and threw it in the dustbin. Why do you think he did this?