The known or estimated year in which the Source was written.
1: content
Type= string // Required= Yes
Contained In:
<attribution> ... </attribution>
Required: No
Repeating: No
Contains Character Data: No
Example of Use:

<date_range content="A.D. 1177"/>

Copyright © 2004, Louis W. G. Barton
Copyright © 2002-2003, The President and Fellows of Harvard College.  Contains software or other intellectual property licensed from Louis W. G. Barton, copyright © 1995-2001 by Louis W. G. Barton.