http://www.rfc-editor.org/rfc/rfc7953.txt
RFC 7953: Calendar Availability, C. Daboo, M. Douglass2016-08-01T23:00:00-00:00This document also defines extensions to the Calendaring Extensions to WebDAV (CalDAV) calendar access protocol (RFC 4791) and the associated scheduling protocol (RFC 6638) to specify how this new calendar component can be used when evaluating free-busy time.