Test with other calendars

[florianhug]

We have no idea if the component works with a calendar other than the gregorian. This should be tested and documented.

Describe the solution you'd like
I want a description of how to configure the component to use a different calendar. The documentation should mention whether different calendars are supported. If not, which is probable since it was never even considered until now, there should be Issues for adding support for other calendar systems.