According to Lifehacker, Granny is using (and way hooked on) the best calendar app out there. If you're wondering which one is best, you can quit wondering: the folks who read and respond to Lifehacker's polls are the savviest around. And their consensus is GoogleCal.
(Note that if you have to use Outlook for business or other purposes, you can sync to GoogleCal. It's worth it.)
Hive Five Winner for Best Calendar Application: Google Calendar