you can install some wp plugin to randomly (or by some rule) reorder wp posts. probably some plugins have option to set time also
with cron you can set what to run (e.g. php or java script), but in that script have to be written what to do
here is some example of custom made:
http://www.daniweb.com/web-developme...ly-random-post
but there is million of plugins for random posts:
http://wordpress.org/plugins/tags/random-post
find one which suits you most, probably there is something what you are looking for without need to write new or if you need to customize some plugin, contact me, I can help you with custom solution