A standalone calendar is rather useless, so it needs to support the most prevalent and open standards for exchanging calendar data:
- CalDAV for syncing to the cloud (or your own server): https://en.wikipedia.org/wiki/CalDAV http://caldav.calconnect.org/ - iCAL for fetching calendars from the web (or importing calendar data) https://en.wikipedia.org/wiki/ICalendar https://tools.ietf.org/html/rfc5545