Right, so creating a countdown clock for forum posts and signatures.
iFrame code and Ajax code will not work on the forums (2 common ways to embed dynamically updating content). BBCode would only give us a static image that refreshes every time the page is refreshed. What you are looking for here is Flash code. Flash code used to work on the forums while using the "embed media" button, but it no longer does. Here is an alternate method while using the same code.
It's simple really. Go to any website that lets you create and customize your own Flash countdown timer (I used http://flash-clocks.com/countdown-generator.html). Most of these will provide you with a Flash HTML code when prompted. Simply create your timer, click the "provide HTML code" (or whatever it is) button and copy and paste the shown code where ever you want to place it (be it a post or a signature).
Voila! Fairly simple stuff, we just needed to do the job of the embed media button by copying and pasting the code ourselves. Please note however that only Flash countdowns will work.