An easy option is VaultWarden. Pretty painless to host AND it’ll store your passwords. There are probably better dedicated tools, but it’s functionality is pretty solid.

I haven’t used it, but agendav - https://github.com/agendav/agendav looks like it might be what you’re looking for. There are similar projects for iCal format, which many calendar apps can emit as well.

It’s disappointing that this is the highest voted comment on a thread in the selfhosted topic…