More details on the security model behind Proton Calendar can be found here. However, once the feature graduates from beta status to stable ProtonMail say that a free, basic version of Proton Calendar will be available for everyone to use, albeit with some limitations.

It’s not clear if free account users get instant access - so if you go through the effort and still don’t see it, apologies! Oh shit, I didn’t know it’s available in other regional App Store as I just found out it’s available in Hong Kong today. If you’re subsisting on a free ProtonMail you can ‘sign up’ to try the ProtonCalendar which, as stated, is still in beta (i.e. You can also use the self chat in Telegram trough Protondrive is obviously going to be better for sensitive or private stuff. If you’re have a paid ProtonMail account you can access ProtonCalendar right now via the ProtonMail v4 Beta portal. The team say there are ‘still debating whether or not there will be ’ to link to other clients. No word on whether it’ll be interoperable with existing calendar clients in the future.

More features are likely to arrive between now and its public release. It currently supports the creation and deletion of events, including recurrent or repeating ones, and lets you set reminders.

Protonmail calendar app