Meaning of "older than"? [Was: Backing up mailboxes]

[Adam Tauno Williams]

On Thu, 2011-06-09 at 10:04 -0700, Andrew Morgan wrote:
> On Thu, 9 Jun 2011, Diego Ventrice wrote:
> > I'm planning a backup strategy for a new server that will handle
> > Hundreds of mailboxes. Largest server for me to admin so far
> > Would you share your experiences backing up cyrus mailboxes ?
> > What tool/s do you use ? how many backups a day do you run ?
> Enabling delayed delete and delayed expunge will help a lot:
> delete_mode: delayed
> expunge_mode: delayed
> You'll also want to modify your expire line in cyrus.conf to something 
> like this:
>    delprune      cmd="/usr/local/cyrus/bin/cyr_expire -E 1 -X 7 -D 7" at=0100
> -D and -X control how long deleted and expunged items remain on-disk:
> 
>     -D delete-days
>            Remove previously deleted mailboxes older than delete-days (when
>            using the "delayed" delete mode).

"Older than" is a confusing statement.  According to my notes this means
a message is kept for a minimum of X days from delivery, not X number of
days *after* it is deleted.  I don't know why my notes say this, but I
remember testing it and convincing myself it is true.  So -X / -D may
not do exactly what you are assuming they do.

Am I wrong? (and are you certain I'm wrong?)