CalDav, so I can use this with our family calendar on synology
To add many other calendar providers we need CalDav to this app.
Only then we can use synology calendar and be less dependent on google
You can sync with any partner supported by Android. CalDAV is one of them.
Max Power commented
You can use the App OpenSync for CalDav sync. Calenders then will be useable in BusinessCalender.
This works very good for me.