I use Thunderbird with Lightning for calendar. Integrating thunderbird calendar with GNOME was possible. I am not sure about GNOME shell.
You can give this a try - http://dmtg.me.uk/2011/04/17/thunder...ar-in-gnome-3/
If you execute
gsettings set org.gnome.desktop.default-applications.office.calendar exec thunderbird
You will be able to open thunderbird from the calendar (basically changing the default application). But it does not show appointments - for me at least.
---------- Post added at 02:15 PM ---------- Previous post was at 02:12 PM ----------
Now that I think about it, I think
, if you setup online accounts, you can use calendar from your online account - google for example. I have never tried this though and so I do not know if this is indeed the case.