You were just wrong. They explicitly gave the understood options as 15 before or 15 after. These are the options everyone uses in English -- not 45 before or a quarter of the hour before. In English no one says quarter of 12 to mean 11:15. You just explained it completely different from the ways it is interpreted in English. I understand the logic and how it might come about. Maybe it's very common in German, but it is not used that way in English. If you referred to 11:15 in that way to a native English speaker you would be misinterpreted.