Synching a Mac Addressbook with NextCloud is not as easy as it might seem.

Theoretically the CardDAV CalDAV Standards should make things easy. In practice it's a nightmare. See e.g.

• https://help.nextcloud.com/t/getting-nextcloud-to-work-with-mac-os-x-adress-book/2269/21
• https://unterwaditzer.net/2015/kill-webdav.html

Here you'll find some technical details that might help you debug your situation.

Links

• https://github.com/andrewlkho/icloud-addressbook-query/blob/master/icloud-addressbook-query.py
• https://apple.stackexchange.com/questions/222619/using-sqlite-browser-to-extract-osx-contacts

Timestamps

• https://www.epochconverter.com/coredata

Database schema