the iCalendar rfc shows nov 1998 - https://www.rfc-editor.org/rfc/rfc2445

iCalendar is based on vCalendar, http://www.imc.org/pdi/vcal-10.txt - need to go to archive.org to see it, which shows Sept 1996.

from wikipedia, XML is listed as first published in Feb 1998, JSON is early 2000s.

edit: XML'd iCalendar, 2011 - https://datatracker.ietf.org/doc/html/rfc6321

JSON'd iCalendar, 2014 - https://datatracker.ietf.org/doc/html/rfc7265