Google Calendar mode sets up a number of fallthrough keys to allow you to use Google Calendar's keyboard shortcuts.
Why can I only see the minimalistic mobile Google Calendar page?
As of no later than 2014-11-24, Google Calendar has started doing something called "browser sniffing" which mistakenly identifies Conkeror as a mobile device's browser. The result is that Google serves up the minimalist version of the Google Calendar page, a version which is effectively read-only, meaning you can't add or edit appointments.
See also changing your UserAgent string per-site.
This workaround reflects the difference between truly free (as in freedom) software, such as Conkeror, and software which does not respect your, the user's, freedom, such as Google Calendar.
With free software, nobody can stop you from fixing it.