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
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