Thread:Trellar/@comment-4544974-20121110005446/@comment-4674838-20121110021239

There are two different already-existing scripts which provide such a feature that I am aware of. DisplayTimer adds the clock to the Wikia header at the top of the page, as seen on the Runescape Wiki, for instance. DisplayClock is similar. Both clocks are in UTC time (Wikia's servers and the badges/achievements system are based on UTC time, so UTC is the most helpful timezone for all editors to be aware of).
 * DisplayTimer
 * DisplayClock

Installation instructions are on those pages I linked. All you have to do is copy/paste the code it says into your wiki's MediaWiki:Common.js page.