For what OS?

Why not just run your own calendar server, then the sync issue is resolved by extant sync mechanism, rather than trying to make your own with Syncthing?

I use Syncthing to sync some stuff that doesn't have standard server sync solutions, like some text files that may get updated at either end. But I don't think it's the way to sync calendar stuff, as calendaring is an established system.

And if you're worried about getting to your calendar server, Mesh networks like Tailscale can provide an encrypted tunnel. You don't even need the client on devices, if you use the Funnel feature (which funnels specific traffic from the internet into your tunnel, courtesy of Tailscale).

(That said, I'm curious to see what more knowledgeable people come up with, I can't think of any calendar apps that auto import/export calendars).

I don’t have an answer to you, but I have a question. Why are you trying to recreate CalDAV? That’s one one of the oldest solved problems on the internet.

You can use DecSync!

synchronizes RSS, contacts, calendars, tasks and more without requiring a server

It is quiet easy to set up for Android with DecSync CC

With any Calendar App you like. I enjoy Fossify Calendar.

For PC you can install DecSync-Plugin for Radicale. And then import your Calender's to any CalDav compatible program.

I really wanted this, but couldn't find anything than worked well. I ended up using tasks.org, an open source todo list that has great calendar functionality and syncing, and moved all my calendar events to it.

ntodotxt is great for a task list that is just a txt file and easy to sync.

For calendars I really think hosting your own would be less work in the end.

https://radicale.org/v3.html

Radicale is super small but is a server..

merkuro

I ended up using khal/ikhal. They're CLI programs. I haven't found a GUI one to work this way.

Radicale with CalDAV is what I use, combines well with Tailscale but does need a server to run on