Buongiorno can also mean 'good day' in English (American or British) if it is said at 1:00 (1300)in the afternoon when it could not be 'good morning'. I wouldn't think 'buona sera' would be appropriate until at least 4:00 in the afternoon (1600). So I don't accept the correction I got that says it is only "good morning'.
While it's true that not everyone speaks American English, it's also true that some people do speak it. And since translations from one language to another rarely done exactly word-for-word, the idea or concept is what matters. Therefore, while the literal "Good day" translation should be accepted, the very common (and for many of us, the predominant) "Hello" should also be accepted.