Let’s face it, the documentation on the MailChimp site is paltry, at best. I’m going to add things here as I come across them.

First up, full documentation for the DATE merge tag.

Usage
*|DATE:FORMAT|*
Format Options
See the PHP Documentation for all the character codes you can use in the FORMAT string. (Thanks for the tip, Chad!)
Examples
*|DATE:l F jS, Y|* = Monday January 15th, 2009
*|DATE:m/d/y|* = 01/05/09
*|DATE:D j M Y H:ia|* = Mon 15 Jan 2009 05:53pm

Please let me know in the comments if you discover anything is not as documented here (or figure out anything that I haven’t!)