Comment 4 for bug 1308296

GNOME does allow defining a default calendar (System Settings -> Details -> Default Applications), so that could be used by indicator-datetime as well. But as Charles pointed out, since indicator-datetime is displaying calendar events, it should probably be keyed to a calendar application that can display those same events as well. If indicator-datetime is opened up to "any" calendar app, like Lightning, then I would think the user experience demands that indicator-datetime have an internal interface into the application's known events, i.e. liblightning, if such a thing existed. I suppose a D-Bus interface could be spec'd out, sort of like Messaging Menu, but I'd be happy with just using the .desktop file as a first step. (To be clear, I'm not wedded to my proposed key names, I was just being verbose for clarity's sake.)