UTC Time zone handling broken -> double entries in calendar

When using a cck datetime field with optional to date (optional repeat) and time zone handling setting UTC there are double entries in the calendar. One correct entry and a second entry with a wrong start time. (If the optional "to time" is set the "to time" of the wrong entry is the correct one. Else it is the starting time. E.g. Node with time 10:00 and to time 10:00 -> displayed as 22:00-10:00,
Node with time 14:00 and to time 18:00 -> dispalyed as 22:00-18:00 (just see screenshot)).
Not completely sure it is a date bug (could be calendar too) but setting the datetime field to "no timezone conversion" fixes the problem (clean calendar, no double entries).
(Note: I don't get the timezone handling anyway: When creating a new node the time is stored as I enter it regardless if I set timezone handling to UTC or "no timezone conversion (checked the database records). I'd expect that the date is stored in UTC in the database (that's what I want) and displayed in the sites timezone user configurable timezone disabled))