Attach different CSS styles on a node basing on publishing date or other PHP properties
You are asking the wrong question. The question you should have asked is "how do I add a class on a node depending on some properties?". Using different set of CSS files to control styling a an element is just a no-no.

The answer to the correct question depends on whether you are targeting Drupal 6 or Drupal 7:


On Drupal 6, you will generally have to implement the logic your own node template;
On Drupal 7 (or on Drupal 6 if you are using an intelligent base theme like Zen), you would implement a hook_preprocess_node() in your theme and add classes to the 'classes_array' variable