Quick reply...
I'm going to try new Qalendar, for several reasons. First, it doesn't seem like it's going to corrupt my Calendar database - not that it's large. Second, it's open-source, and therefore can be more easily modified-extended in the future. Third, I want to leave my feature requests. Such us, make sure that Qalendar doesn't use too much memory or processor, or react too slowly - default Calendar is one of the fastest opening applications. Don't include sync inside your application - sync should be an optional extension, not a vital part. Fourth, I do have an event happening on Friday 13th, which should be added to my diary, and your recurrence editor is an excellent feature.
Best wishes.
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Per aspera ad astra...
Written and posted from MicroB.