OpenACalendar is an Open Source collaborative group wiki calendar, with strong Open Data tools baked in.
We can let any user add and edit events, like a wiki, or only certain users can have edit rights. All edits and deletes are versioned to ensure safety, like a wiki.
There are feeds and an API so exporting the data into the system of your choice should be easy. It handles multiple countries and/or time zones on the same calendar.
One installed base of this system can run one calendar, or run several calendars.
Open Tech Calendar was started in July 2012 after a similar calendar listing tech events run by Tech Meetup was accidentally destroyed. We set out to create a calendar that was more useful, could welcome more contributors and crucially, was less likely to be accidentally destroyed.
The first version, a Mediawiki site with a plugin, was launched in 4 days.
Several months later the second version, a custom written webapp, was launched.
Over winter 2013/2014, a new version was written that lets several calendars be run from the same install. From this new codebase, Open Tech Calendar was switched over, HasACalendar was put online and OpenACalendar was released.