By clock I mean when you set alarms ... I guess the calendar belongs to the same category.
I can refine this by saying;
1. Phone and SMS from external sources and strangers.
2. Clock, Calendar, etc for local personally chosen alarms.
3. Email for everything else coming from apps and services etc.
A calendar can belong to #3, but you don't need a backend for the calendar app, as you said it is similar to a clock on your device.
