Review for Purge Events Plus by Just1
Rated 3 out of 5 stars
A must-have for everyone who wants to keep its calendar(s) clean. Particularly necessary when sharing calendars with friends/collegues (not to keep traces of the past for newcomers).
To me, it lacks an automatic function to clean periodically (not only manually). In that way, each calendar must have its own "older than" option. By the way, the "older than" field could become manually customizable (not from a pre-determined list).
To create your own collections, you must have a Mozilla Add-ons account.