Is there a Linux calendar app that works well with OwnCloud and doesn't depend too heavily on any single desktop environment?
You could also use this : https://apps.owncloud.com/content/show.php/Calendar?content=168707
I know it's not a desktop app but it shows the calender on any platform you want 
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Thunderbird + Lightning extension. Works probably on most platforms. However, sometimes I had experienced some strange sync problems that I mostly solved by deleting and re-adding the sync.
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Thanks tflidd, Lightning looks nice. I couldn't add the "main" calendar address, had to add individual calendars, but fortunately I don't have that many …
kloknibor: I already use the web one but it doesn't (yet) do notifications