I found my problem.The problem was that I put a slot(changedTime) at any QTimeEdit element. I already change the time by selecting the time zone with the radio button.
So after I slected my time zone every time of every QTimeEdit elements are changed. And after that all the functions of every changedTime-Slot tries to change the time of each other QTimeEdit elemts until the application doesn't respond anymore.

Now I have to find a slot which just responds if i clicked a key(for example: enter-key).