Ask HN: Self Hosted Calendars?

I am aware of self hosting email, but what about self hosting calendars?

I would like to be able to send calendar invites between different users of domains and be able to sync different events.

For example, userA@gmail.com sends calendar invite to userB@selfhostedemail.com.

At some point userB accepts or denies the calendar invite. UserA is subsequently notified of UserB decision.

I have seen this work well within users of the same domain (particularly, o365 at work). To some degree, have seen it work between o365 and google workspace users.

Not sure if this is a standard or if these services have some proprietary software to get it to work.