Consider:

• Admin needs the retention periods to be dependent on when content is created, not a course date which mostly in moodle sites isnt set, and may never be set.

Other possibilities include:

• examine logs for activity (not necessarily reliable)
• add a log event watcher which updates a course field (only suitable for data going forward)
• per-activity calculation (e.g. assignment due date, quiz close date, etc).

Potential issues:

• logs are not guaranteed
• does the act of viewing the course extend its lifetime (Heisenburg)