What is the “Date” header?

The Date header shows when the message was composed. The sending client or server writes it in a standard format.

This header can be faked by changing the sender's clock. So for serious logging or legal review people trust the Received headers instead. Those are added by each server that handled the message. They are like stamped port entries on the route of the ship.

It is the timestamp in the captain's log. It tells you when the message left the pier.