How do I add a date to a signed document?

Are you asking how to do this when Sending an envelope or when Signing? Basically DocuSign has a "Date Signed" field that can be added to a document during the envelope Send process. It fills out automatically based on the Recipient/Signer. In other words it a DocuSign generated timestamp. You can add the field as a Sender, add it during a Correct, or request that there is a field placed if a Signer.

If you are looking for a generic field that is filled out by the Recipient (not auto-generated), then use a Text box and set a Validation of "Date" which ensures correct date information in this field.