They used to have a HTML code you could put in your signature so the countdown would show up. Now HTML has been disabled so a lot of people (like myself) use a program such as tickercentral.com or tickerfactory.com (click on mine below and it will take you there) and post the non-html code (I used the bbCode in mine) in their signature. If you have more questions there is a technical board where those can be answered: http://www.disboards.com/forumdisplay.php?f=43
I like the one in my signature better. It gives you the option of counting down in days, and I much rather see the days when I'm this close to a trip!!!!!!!! Just click on the one in my siggie and it will take you to the site.