If you're ok with hacking K2, you can modify the date format of the Extra Field Date:

File:
/components/com_k2/models/item.php

Find:
$value = JHTML::_('date', $value, JText::_('K2_DATE_FORMAT_LC'), $offset);

Replace K2_DATE_FORMAT_LC with the format that you desire, for example: l d F Y [or] mdY

Check this article by Steven Johnson (Intown Web Design) for date options/ideas:
www.jbeginner.com/tutorials/extensions/how-to-format-the-k2-date-time/

Hello Joe,

This is actually a core file edit.
It is best to use language overrides, since you will lose your edits when you update.

I know hacking the core is bad, but wouldn't changing the Language File as you suggested create a "Global" change?

In my case (as possible with others) I only want to change the date format for the extra fields and nowhere else.

Hmm, I have to agree with you.
Changing all other dates will take too much time.