Thank you very much fglock. Two more questions if you don't mind :-)
First: would you generally recommend to use DateTime::Event::Recurrence and DateTime::Event::ICal
instead of DateTime::Set ?
Second: not to promote my hack :-) but just to understand the things better: the code of the OP seems to work with DateTime::Set if the end point is defined ( Re^3: Determine if a given DateTime is a member of a DateTime::Set ) - is it just a coincidence? do you think it will break under other constellation?