http://us3.php.net/date , but that's just the manual for formatting date strings in PHP.
Hope that I'm not belaboring the obvious here, but the code that actually formats the date string isn't in the code you pasted. It's in the date_if_changed() function of the parent Class of the $Main_List object. I'm assuming you have an include() or require() file on that page that contains those functions. I'd check there.