Each release they say they've fixed something in CalDAV yet my simple test of adding an entry on my Mac, which syncs to CalDAV fine never makes it to my Jolla, makes it but is 1 hour out, disappears on the next sync or vanishes when you edit it on the Jolla. It's got better - at one point you couldn't even get a list of calendars. I watch the github buteo_sync pages hoping that one day there's a bug fixed for "Apple's CalDAV" or similar but there's never that much activity. It doesn't seem to be high priority even though it was IIRC top three on TJC before they declared it "implemented". I don't know which CalDAV implementation it supposedly works on but it's not OwnCloud, SabreDAV or iCloud.