Glossary
CalDAV
CalDAV is the open standard for reading and writing calendar events over HTTP. It is what lets Apple Calendar, Fastmail, Yahoo Calendar, and self-hosted servers like Nextcloud expose events to third-party clients.
What CalDAV is
CalDAV (Calendaring Extensions to WebDAV) is an IETF standard, defined in RFC 4791, that lets clients read, create, update, and delete calendar events on a remote server using HTTP requests. Apple iCloud, Fastmail, Yahoo, Zoho, and most self-hosted calendar servers expose a CalDAV endpoint.
Why it matters for sync
Google Calendar and Microsoft Outlook use OAuth and modern REST APIs. Most other calendar systems do not, but almost all of them speak CalDAV. A sync tool that supports CalDAV can integrate with the long tail of providers, including iCloud, without each provider needing a custom API client.
Authentication
CalDAV servers vary in how they authenticate. Apple iCloud requires an app-specific password generated at appleid.apple.com. Fastmail and Yahoo also use app-specific passwords. Self-hosted servers typically use a username and password over HTTPS.
How SyncCal uses CalDAV
SyncCal connects to any CalDAV server using the credentials you provide. Once connected, the CalDAV calendar behaves like any other: you can mirror events to or from it, apply filters, and use two-way sync.
The most common CalDAV setup we see in practice is syncing Apple iCloud with Outlook, since Apple does not publish a native Outlook integration that works across Mac, Windows, and the new Outlook clients. Syncing iCloud with Google is the second most common.