Ask HN: GNU/Linux calendar software handling CalDav delegation and invitation?

Both Thunderbird (w/ TbSync) and Evolution work fine for basic calendar stuff.

However, when comparing to Apple Calendar, they seem to lack two crucial features:

- calendar delegation (sharing with other users)

- meeting invitation; the events do show, but I can't way to change one's attendance status

It seems to me that at least one software capable of doing this should exist. Am I missing something? Is CalDav just a broken protocol whose full implementation is doomed to remain just a sweet dream?