5
It is in language\english\global.php, lines 160 to 162. It looks similar to this (this is of course my own settings - see my site to see how it looks):
define("_DATESTRING","d/m/Y G:i:s");
define("_MEDIUMDATESTRING","d/m/Y G:i");
define("_SHORTDATESTRING","d/m/Y");
Read the lines that follow it (164 - 190) to choose what settings you want. Make a backup before changing anything though!!
I also had to make changes on my site and was surprised it was not an admin feature. Good to see it will be incorporated into the next release. Bravo