I feed my Mozilla calendar through iCal from basecamp, what seemed to work fine. Today I realized that milestones disappear from my calendar as soon as they get marked completed. However in a calendar application I need to know what there was, not only what to come.
The best thing to solve this issue would be to add status information to each iCal-item. So a completed milestone should change its state from due to completed in the iCal-feed instead of simply disappearing from the list.

+1

I’d like to keep them in the calendar as a reference.

Sounds like a good idea to me so +1