That is not a change in Domino 6. It is a traditional peculiarity of Notes.
It has nothing to do with LSGMTTime or zones. When you display a date/time value, the default conversion to a String removes the time if it is exactly 00:00:00. Cstr(Cdat(23456)) reproduces it. If you use Format to specify the conversion, it does not happen. Alternatively, add a centisecond when the time is zero: Cstr(Cdat(23456 + 1/8640000))
By the way, DateOnly and TimeOnly are not "components". They are also String conversions. TimeOnly does format the time as "00:00:00" when I try it.