International date format
U.S. date format
Unzip the file and then choose “Add Group From File…” in the plus (+) menu under your list of snippets.
You could also link directly to the groups on our server. Choose “Add Group from URL…” and enter one of these URLs:
International: http://www.smileonmymac.com/te/Dates.textexpander
U.S.: http://www.smileonmymac.com/te/DatesUS.textexpander
Thanks to Mar for sharing these. As he notes, they are modifiable, so if you have your own special date needs–and are not afraid of a little code– download the file and customize it!
Or better yet, create a new and exciting snippet group and share it with the rest of us. 
(Send them to me at jean@smileonmymac.com.)