CalDAV support comes to Google Calendar
www.tuaw.com/2008/07/28/caldav-support-comes-to-google-calendar
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Well, fortunately, Google has just quietly introduced CalDAV support to Google Calendar. CalDAV is the protocol that iCal uses to transmit data over the web. Although some other mail and calendar programs support CalDAV, right now Google Calendar is only compatible with iCal. Finally, iCal and Google Calendar can sync without having to use third party programs!