You need to add a field to the Links table "expired" and make it an Int or small Int.

The rest of the use would be in your templates by using <%if expired%> tags.

If you want the ads deleted when they expire, this should be part of the cron script.

It needs to:

delete from Links where Expired=0

and run right after the cron script that decrements the date.

Update Links Set expired=expired-1 where Expired > 0

In your templates, if <%Expired%> is greater than 0, the ad is not expired, so
it actually works backwards -- you might want to rename the field -- but I
prefer just using a more complete test --

<%if Expired >0%> rather than the short hand <%if Expired%>