2
In xoops-root/language/[language]/global.php
Near the bottom of the file find this:
define("_DATESTRING","Y/n/j G:i:s");
define("_MEDIUMDATESTRING","Y/n/j G:i");
define("_SHORTDATESTRING","Y/n/j");
Change the format to suit. A list of the different formats is provided there too.