I made minor modifications to the CSS provided here to style an event countdown ("Only 5 days until...") for HK Activists. The HTML is simple; we just need the selectors for the CSS to act upon. Some of the CSS is newfangled CSS3, and the display looks fine without CSS3 too.
The display has been (briefly) tested in the latest versions of Safari, Chrome, IE, and Firefox as of January 2012.
My HTML (note that "<?php print $daysuntil; ?>" should be replaced with your number or variable outputting the number of days left):