Hi Christian,
Don't get me wrong, but I think you'll just be able to send date or time information but not both of them combined because of the 3 octet encoding. Probably you'll have to send the date and the time information to different GAs.
The item configuration looks ok to me, but how do you update the item state? I've configured my item with additional NTP-binding to update the GA for the timesource automatically.
DateTime Date "Datum / Uhrzeit: [%1$tA, %1$td.%1$tm.%1$tY %1$tT]" <calendar> { ntp="Europe/Berlin:de_DE", knx = "10.001:x/x/x" }
Hopefully somebody else will post a more convenient answer to your question.
Good luck and best regards,
Michael