I run algebra.com with a few thousand registered users and a lot more
unregistered. I use Apache::Session to store their session info. Right
now I clean sessions every month. I would like to be smarter and clean
sessions based on date information (ie remove sessions that are more
than 3 weeks old).

Can I have a date as a relational column in the sessions table?

I know that I could maintain a date in the session data blob, but, it
is expensive to use that for a few reasons.

Right now it's been a couple of weeks, and I have 142,000 sessions.