Right now, there is no way to make your calendar available to other apps (like an app for a smartwatch), so they could see the events from the proton calendar.

The app should use CalendarContract API to make it happen, optionally registering itself as a sync provider. This will allow other apps to fetch calendar entries from Proton calendar just like it is now possible with Google Calendar or Microsoft Exchange. This will make it possible to see your timeline on your smartwatch if the app for it supports this feature.