If you do not already know it, here's a tip for automatically inserting a modification date:
http://www.newbie.org/snippets/ssi/c...flastmod9.html
For SERPs, however, this does not have any influence. What *might* count - I do not have any evidence, but it seems logical - is the "last-modified" and "expires" header info sent by your server, indicating how "fresh" your information is. See an introduction at
http://www.mnot.net/cache_docs/
Even if it does not (currently) help with the SEs, it is a good idea to provide those header data, since it is good for caching.
hth,
Alex