How to mirror Older files only?

Hi,

I've been trying to figure out how to do this, and hopefully I'm missing 
something obvious.  I'm looking to archive files from an FTP site.  What I 
would like to do is FTP all files older than a particular date to my machine 
where I can transfer them to a backup medium.  At the same time, I would 
like to delete the files from the FTP site as they are no longer needed 
there.

I've been racking my brain for a while, but haven't been able to come up 
with an obvious solution to this problem.  I haven't found any flag I can 
set in the mirror command, and no other simple solution.  How difficult 
would it be to add a -O / -older-than=SPEC flag to the mirror command where 
it would only FTP files older than a particular date?

If not, can anyone provide me with a solution to try?

Thanks!

Eric