U-Boats normally did not change their clocks as they moved into different time zones. They stayed on German War Time which is Greenwich Mean Time plus two hours. Then and now Germany is on Central European Time which is GMT plus one so GMT plus two is year ’round daylight savings time.
Charles McCain is a Washington DC based freelance journalist and novelist. He is the author of "An Honorable German," a World War Two naval epic. You can read more of his work on his website: http://charlesmccain.com/ View all posts by Charles McCain