Wikibase (thus Wikidata) is able to store the required information. The time portion of dates might need some love first, though. It is able to provide that information to multiple clients (all Wikimedia wikis). And to external consumers in various formats. (Perhaps query.wikidata.org needs support to output ics! Sometime after exposing results to Lua.) Maybe conference occurrences can be entered in Wikidata (like was done for Wikimania 2016 https://www.wikidata.org/wiki/Q19291990 ) and maybe that could then link to their ics URL of the schedule...

SMW is as the links above show able to store the required information. AFAIK as is, it is not able to provide that information to other wikis than the wiki where it is stored, though it can probably provide it to external consumers in various formats. Would be useful if SMW and Wikibase supported a common query result format. So that they could share many extensions. We didn't update the version of SMW that is running on one of the Wikimedia wikis. See e.g. https://github.com/SemanticMediaWiki/SemanticMediaWiki/issues/1479 (could need some love independent of this task).

The Phabricator calendar doesn't automatically subscribe me to materialized instances of recurring events. (That reminds me that we have tasks that we manually repeat where I sometimes also need to subscribe to the next one manually depending on if the subscribers were copied.)
> I oppose spending money on this unless we first spend money on the Phabricator upstream bugs which Wikimedia users suffer from.
Yes #Phabricator-Upstream has some that are more important than those related to Calendar.

Anything that doesn't export caldav/ical doesn't help me personally (all people I know use devices and software that can consume CaldDav or ics files from the Internet).
Special Google Calendar support is probably not needed as google supports ics.
Ext:Cargo sounds interesting. Other back ends (than SMW) could be supported in the same way.
I personally use https://en.wikipedia.org/wiki/DAViCal as a back end.
Still need to find something to sync ics files to a CalDav server. And something to filter and aggregate calendar events ( like query.wikidata.org ? ), e.g. to get an ics file with all events related to open educational resources (Q116781) that are at most 10km from the center of Berlin (Q64).

Does FullCalendar support CalDav?
http://agendav.org/ looks interesting as a front end.