The DATE_FORMAT is defined in your Joomla language file as being %A, %B %d, %Y
%I:%M %p – or, for example: Tuesday, March 14, 2011 08:30 AM.
I believe the language file is located within –> administrator/language/en-GB/en-GB.com_k2.ini
(or something to that effect)
You can take a look at your current language file (language/en-GB/en-GB.ini) and at the top you will see the date formats laid out. If one of those options will work for your site, then all you need to do is override the K2 default template (or create a new K2 template).
Perhaps the following resource may be of additional assistance to you . . . How To Format Date/Time in K2