From Sonspring.com, there's a quick tutorial on a simple way to make "time-sensitive CSS" for your site.

Recently, there's been some discussion on the Godbit forum about how to serve dynamic, time sensitive CSS with PHP. I got to thinking, and the steps necessary to make this work are actually pretty easy. First off, let me say that most hardcore programmers will probably scoff at the simplicity of these examples. However, a hardcore programmer I am not, so basic tutorials are what I enjoy. Also, please know that this is not an exercise in JavaScript / Ajax. It is simply a way to provide unique CSS based on date and time.

They include examples of what the result of the code is for two different methods - the month-based CSS and the PHP + CSS Clock approaches. Their examples are just the sample uses of the code, but they also make each available for download. Check out all of the comments for even more tips!