CaptainCasa offers the great possibility to define the client time zone centrally by means of t:clientconfig or sessiondefaults.xml. This way, the default timezone must not be set for the particular components.
But now I have noticed that ExtCalendar (used by means of "excalendarid") is not effective if the attribute "timezone" is not explicitely set on the component t:calendar. In this case the calendar does not show any special information defined on server side.