systemd journal retention time setting not working

[Chris Green]

To try and reduce the ridiculously large systemd journals on my
systems I have set the following in /etc/systemd/journald.conf :-

    MaxRetentionSec=1week

However it doesn't seem to be working, I've just found that I have
journal files which are months old.  I've done this on a few systems
and it's not working on any of them.  They have all been rebooted
since I changed the setting.  Is there anything else I need to do (i.e.
some systemd command) to get this setting to take effect.

-- 
Chris Green