The URL of the mail app (https://runbox.com/app/) may be bookmarked or accessed from history. Accessing the Calendar app in a similar way yields an error.
The “location” in the calendar app (week or month view, which month, which event currently open; holds for the mail app too [except the Draft folder, https://runbox.com/app/compose]) is not reflected in the URL. I see this as an inconvenience.