Thanks for the hints, guys.
Ok, so I can set start time already. 😀 Excellent!
Then as an addition and to make setting the start date/time more intuitive:
How about changing the button for entering a break time (in german it's "Pause einfügen") for the starting point to a button for entering starting time ("Startzeit festlegen")? A break at the start seems to make little sense to me...