Ask HN: What do you use to work with calendars and events?

I will be working a project that will serve as a calendar for small-ish teams. The prototype was built on top of the Google Calendar API, but for the actual project I have to consider other options, mostly for privacy related reasons. Apple CCS has been archived earlier this year, so I am curious what other solutions there are and what your experiences have been. Is DIY with Postgres an option, or should I avoid that entirely?