https://partofthething.com/thoughts/host-your-own-contacts-and-calendars-and-share-them-across-devices/
Is an award-winning open-source CalDAV/CardDAV synchronization app for Android 4+. https://davdroid.bitfire.at/
origin - http://www.pipiscrew.com/2017/07/host-your-own-contacts-and-calendars-and-share-them-across-devices/ host-your-own-contacts-and-calendars-and-share-them-across-devices