Archiving old mail

I'm only just starting out with procmail, having found an OS/2 port in
the last couple of weeks, so please be gentle with me...

How do I set about archiving mail by date, either month or year?

There are thousands of emails that have been built up over the years,
whilst I have been looking for a good filtering tool, but now that I
have one, I can't work how to write a recipe to create folders based
on dates.

Any help would be appreciated.

Many thanks,

-- 
John