There's a Chrome Extension for Google Calendar called "Checker Plus for Google Calendar." Among other features it places a calendar icon with the day's date in the Chrome toolbar, just like the icon on the right of the Proton Mail app. However, it also overlays a box with a countdown to the next event ("2 days," "1 day", "6 hours," "44 min," etc.)

It would be fantastic to have something like that overlayed on the Proton Calendar icon in the app. It helps me remember that I've got a thing coming up.

You can get a sense of it in this video, near the end. https://www.youtube.com/watch?v=h91GsxKACdc