Rated 3 out of 5 stars
1. The author deemed it necessary to force a check on an installed Lightning / Sunbird. Sadly, this caused the extension to break for some of us. This can be fixed by removing the lines 34-41 (everything that's "") from install.rdf (extract, change code, pack all files into ZIP, rename ".xpi", install in TB).
2. There's some cruft "build.bat" stuff flying around in the .xpi - This is unnecessary, unclean work.
3. The extension does not work as I expected: You have to actively select the option to purge events - It does NOT purge events older than X automatically for you (You have to do a right-click on the calender-name in the left sidebar of Lightning to find the option to purge events...).
4. This extension only purges single-time events that happened in the past. Yearly events and the like are still visible in the history of the calendar. It would be great if those yearly events also had their older reoccurences purged.
Despite all these problems, the extension works (after fixing install.rdf) and closes a gap that Lightning should have closed a long time ago!