Technique
Obviously, if xhtml offered a tag which captured this syntactic meaning, it would be much easier to solve this problem.
By adding comments like <!--date.2006.3.21-->
in close proximity to new content, it is possible to place relevant information in parts of an html file and use dom scripting techniques to make it relevant for newer browsers.
Example
Here is an example of how the code looks like in this webpage (so you don't have to 'view source' this page.
<h2>Example</h2><!--date.2006.3.25--> <p>Here is an example...</p>
This kind of code snippet, in this website's case, gets transformed into something like the following:
<h2> <strong title="posted xx days ago">New: </strong> Example </h2><!--date.2006.3.25--> <p>Here is an example...</p>