I thought 'giorno' was 'day' and 'matina' was morning (using Michel Thomas programme, so I can't spell). Guessing I'm just plain wrong though..?
Even though it technically means 'good day', it is usually only said in the morning or early afternoon. 'Good day' isn't used very often in English, so 'Buongiorno' tends to be translated as 'Good morning' as it makes more sense. 'morning' is 'mattina', but I don't think 'Buona mattina' is used as a greeting.